Zoom lens, video enlarging/projecting system, video projector, rear projector, and multivision system

ABSTRACT

The present invention includes three or more lens groups. These are arranged in the order of a first lens group ( 11 ) that has positive refractive power and a second lens group ( 12 ) that has negative refractive power, as seen from the side having the longer conjugate distance, wherein a first lens ( 12   a ) of the lenses of the second lens group ( 12 ), as seen from the side having the longer conjugate distance, has positive refractive power. Thus, it is possible to suppress distortion aberration to a small value.

TECHNICAL FIELD

The present invention relates to zoom lenses, and particularly relatesto zoom lenses used in devices such as projectors for enlarging andprojecting images, which are converted by spatial optical modulatingelements, onto a screen.

BACKGROUND ART

In projectors that use reflecting-type spatial modulating elements forthe three primary colors red, green and blue, prisms for guiding theilluminating light and prisms for combining colors are disposed betweenthe projecting lens and the spatial modulating elements. Because ofthis, the projecting lens requires a long back focus. Since spectralcharacteristics of prisms for combining colors are dependent on theincident angle, it is necessary to have an optical system in which thepupil on the shorter side conjugate distance is a sufficient distancefrom the spatial modulating elements, that is to say, it is necessary tohave telecentricity.

For a forward convex group four-group zoom lens in which the long backfocus and telecentricity do not vary with zoom, there is the zoom lensproposed in Patent Reference 1 below, for example. Furthermore, as aforward convex group three-group zoom lens, there is the zoom lens thatis proposed in Patent Reference 2 below, for example. As a forwardconcave group four-group zoom lens, there is the zoom lens proposed inPatent Reference 3 below, for example.

Furthermore, as a telecentric wide angle lens, there is the wide anglelens proposed in Patent Reference 4 below.

There is also a demand for projectors in which the projection distancefrom the screen to the projector is short and that can be used in asmall space, and there is also a demand for wide angle lenses that canbe used over a short projection distance for projecting lenses.

Furthermore, in the case of wide angle lenses, the manner in whichdistortions are corrected is very important. Aspheric surfaces have agreat ability to correct distortions, and can reduce the outsidediameter of the lens and the number of constituent lenses. As atelecentric wide angle lens in which aspheric surfaces are used, thereis the wide angle lens proposed in Patent Reference 5, for example.

Furthermore, methods are proposed for obtaining a bright image bycombining the image from two projectors onto a screen, and for obtainingan image having a large aspect ratio by lining up two projected screensnext to each other.

However, in such projection methods in which two projectors or twoscreens are used, it is necessary that the corresponding pixels areprojected onto the same position by two projectors. If used under theseconditions, distortions that were not conventionally a problem become asignificant issue.

That is to say, the position of the corresponding pixels that areprojected from the two projectors onto a screen shifts with distortionsof conventional projection lenses, and there is a considerable loss ofresolution. Thus, it is necessary that the distortion of the projectinglens is sufficiently small, however the zoom lens proposed in the abovenoted Patent Reference 1 has a large distortion value of about −2% atthe wide angle end and about +0.3% at the telephoto end.

Furthermore, in addition to the distortions being sufficiently small, itis desirable to have a compact projecting lens that also has a long backfocus. However in the zoom lens proposed in the above-noted PatentReference 2, in addition to the fact that the back focus isinsufficient, the distortion is as large at about −2% at the wide angleend and about −1% at the telephoto end and the overall length of thelens is about 11 times the focal length at the wide angle end, thusposing problems for miniaturization.

Furthermore, in the case of the zoom lens proposed in the above-notedPatent Reference 3, the distortion is as large at about −2.7% at thewide angle end and about −1.2% at the telephoto end, the F number is dimat about 3.5 and brightness cannot be ensured.

Next, in the case of the wide angle lens proposed in the above-notedPatent Reference 4, there is insufficient back focus for use as aprojecting lens for a projector that uses reflecting-type spatialmodulating elements.

Furthermore, in the case of the wide angle lens in which asphericsurfaces are used, proposed in the above-noted Patent Reference 5, thereis insufficient correction of axial chromatic aberration and chromaticcoma (a condition in which there is no coma aberration with respect tothe standard wavelength, but there is downward coma aberration with ared color of 620 nm, and upward coma aberration with a blue color of 460nm). This is because the aspheric surface has no ability to correctcolors.

Accordingly, many wide angle lenses for projectors are used in a rearaspect, and these also may be combined with a backing mirror and used asa single unit. In this case, the lens is projected onto an approximately178 cm diagonally dimensioned screen, and therefore the lens requires acapability at close distances.

However, the performance of wide angle lenses fluctuates greatly withprojecting distance. In particular, as noted above, wide angle lensesfor projectors require a long back focus, and the lenses are arranged ina sequence of concave to convex from the side with the longer conjugatedistance. This is what is known as a reverse telephoto-type (retrofocus-type). In this configuration, asymmetry of the lens arrangementwith respect to the aperture stop increases, and change of performancewith respect to the change of projection distance becomes larger. On theother hand, in the case of a wide angle lens that is symmetrical withrespect to the aperture stop, even if the height of the light beams thatpass through the lens changes, there is little change in the performanceof the lens because the lens is operated so as to compensate for theaberration around the aperture stop.

That is to say, for a retro focus-type lens such as is described above,since the asymmetry of the lens arrangement with respect to the aperturestop is large, if there is a change in the height of the light beamsthat pass through the lens due to a change in the projection distance,then the aberrations are not cancelled out and there is a change in theperformance.

Consequently, it is a significant problem to ensure the performance ofwide angle lenses for projectors, with changes in projection distancewhen projecting onto screens having a size of 782 to 178 cm, forexample.

[Patent Reference 1]

JP H10-161027A

[Patent Reference 2]

JP 2001-215411A

[Patent Reference 3]

JP 2002-131639A

[Patent Reference 4]

JP H11-109227A

[Patent Reference 5]

JP 2002-131636A

DISCLOSURE OF INVENTION

The present invention is to solve the above problem, and it is an objectof the present invention to provide a compact zoom lens that has a longback focus, and that has little distortion, and little lateral chromaticaberration in order to realize a bright, high definition projector. Itis a further object of the present invention to provide a wide anglelens that has a long back focus, that has little distortion, littlechromatic aberration, and whose performance changes little with respectto changes in the projection distance in order to realize a bright, highdefinition projector.

In order to achieve these objectives, a first zoom lens according to anaspect of the present invention comprises at least three lens groupsthat are arranged in an order of a first lens group that has positiverefractive power, and a second lens group that has negative refractivepower, as seen from the side having the longer conjugate distance,wherein the first lens of the lenses of the second lens group as seenfrom the side having the longer conjugate distance has positiverefractive power.

A second zoom lens according to an aspect of the present inventioncomprises a front lens that is a negative lens, as seen from the sidehaving the longer conjugate distance, wherein the followingrelationships are satisfied:−0.018<(1/f1 /abe1 )/(1/frear)<01.7<nd11<1.79

where f1 is the focal length of the negative lens, where abe1 is theAbbe number and where nd11 is the refractive index at the d line, andwhere frear is the focal length of the lens group on the side having theshorter conjugate distance, with respect to an aperture stop.

A third zoom lens according to an aspect of the present inventioncomprises four lenses, as seen from the side having the shorterconjugate distance, wherein in the order from the side having the longerconjugate distance, a negative meniscus lens whose convex surface facesthe side having the longer conjugate distance, a positive lens, anegative meniscus lens whose convex surface faces the side having theshorter conjugate distance and a positive lens, and wherein thefollowing relationships are satisfied:nd4>1.75vd4>401<f4 r/bfw<4

where nd4 is the refractive index at the d line of the negative meniscuslens that is on the side having the longer conjugate distance, where vd4is the Abbe number, where f4r is the focal length of the four lenses andwhere bfw is the air equivalent back focus that does not include a prismand a cover glass when at the wide angle end.

A fourth zoom lens according to an aspect of the present inventioncomprises a first lens group that has positive refractive power, asecond lens group that has a negative refractive index and a third lensgroup that has a positive refractive index, arranged in that order fromthe side having the longer conjugate distance, wherein when changingmagnification from the wide angle end to the telephoto end, the firstlens group, the second lens group and the third lens group move alongthe optical axis, wherein the first lens group moves monotonicallytoward the side having the longer conjugate distance, the second lensgroup moves monotonically toward the side having the shorter conjugatedistance and the third lens group moves monotonically toward the sidehaving the longer conjugate distance, and wherein the followingrelationship is satisfied:1.6<bfw/fw<2.4

where bfw is the air equivalent back focus of the zoom lens at the wideangle end when at infinity and where fw is the focal length of the zoomlens at the wide angle end.

A fifth zoom lens according to an aspect of the present inventioncomprises a first lens group that has positive refractive power, asecond lens group that has a negative refractive index and a third lensgroup that has a positive refractive index, arranged in that order fromthe side having the longer conjugate distance, wherein when changingmagnification from the wide angle end to the telephoto end, the firstlens group, the second lens group and the third lens group move alongthe optical axis, wherein the first lens group moves monotonicallytoward the side having the longer conjugate distance, the second lensgroup moves monotonically toward the side having the shorter conjugatedistance and the third lens group moves monotonically toward the sidehaving the longer conjugate distance, and wherein the followingrelationship is satisfied:1<bfw/fw<1.8

where bfw is the air equivalent back focus of the zoom lens at the wideangle end when at infinity and where fw is the focal length of the zoomlens at the wide angle end.

A sixth zoom lens according to an aspect of the present inventioncomprises a first lens group that has positive refractive power, asecond lens group that has a negative refractive index and a third lensgroup that has a positive refractive index, arranged in that order fromthe side having the longer conjugate distance, wherein when changingmagnification from the wide angle end to the telephoto end, the firstlens group, the second lens group and the third lens group move alongthe optical axis wherein the first lens group moves monotonically towardthe side having the longer conjugate distance, the second lens groupmoves monotonically toward the side having the shorter conjugatedistance and the third lens group moves monotonically toward the sidehaving the longer conjugate distance, and wherein the followingrelationship is satisfied:0.5<bfw/fw<1.3

where bfw is the air equivalent back focus of the zoom lens at the wideangle end when at infinity and where fw is the focal length of the zoomlens at the wide angle end.

A seventh zoom lens according to an aspect of the present inventioncomprises a first lens group that has positive refractive power, asecond lens group that has a negative refractive index and a third lensgroup that has a positive refractive index, arranged in that order fromthe side having the longer conjugate distance, wherein when changingmagnification from the wide angle end to the telephoto end, the firstlens group, the second lens group and the third lens group move alongthe optical axis, wherein the first lens group moves monotonicallytoward the side having the longer conjugate distance, the second lensgroup moves monotonically toward the side having the shorter conjugatedistance and the third lens group moves monotonically toward the sidehaving the longer conjugate distance and an aperture stop moves inconjunction with the third lens group, and wherein the followingrelationship is satisfied:|(DG1 −DG3 )/fw|<0.15

where DG1 is the amount that the first lens group moves from the wideangle end to the telephoto end, where DG3 is the amount that the thirdlens group moves from the wide angle end to the telephoto end and wherefw is the focal length of the zoom lens at the wide angle end.

An eighth zoom lens according to an aspect of the present inventioncomprises a first lens group that has positive refractive power, asecond lens group that has a negative refractive index and a third lensgroup that has a positive refractive index, arranged in that order fromthe side having the longer conjugate distance, wherein when changingmagnification from the wide angle end to the telephoto end, the firstlens group is fixed, and the second lens group and the third lens groupmove along the optical axis, wherein the second lens group movesmonotonically toward the side having the shorter conjugate distance andthe third lens group moves monotonically toward the side having thelonger conjugate distance and an aperture stop moves in conjunction withthe third lens group, and wherein the following relationship issatisfied:|DG3/ fw|<0.15

where DG3 is the amount that the third lens group moves from the wideangle end to the telephoto end and where fw is the focal length of thezoom lens at the wide angle end.

A ninth zoom lens according to an aspect of the present inventioncomprises a first lens group that has negative refractive power, asecond lens group that has positive refractive index, a third lens groupthat has positive refractive index and a fourth lens group that haspositive refractive index, arranged in that order from the side havingthe longer conjugate distance, wherein when changing magnification fromthe wide angle end to the telephoto end, the first lens group, thesecond lens group, the third lens group and the fourth lens group movealong the optical axis, wherein the first lens group moves monotonicallytoward the side having the longer conjugate distance, and the secondlens group, the third lens group and the fourth lens group movemonotonically toward the side having the longer conjugate distance,wherein an aperture stop is positioned within the second lens group andmoves along the optical axis with the second lens group when themagnification changes, and wherein the following relationship issatisfied:2.5<bfw/fw<4

where bfw is the air equivalent back focus of the zoom lens at the wideangle end when at infinity and where fw is the focal length of the zoomlens at the wide angle end.

A tenth zoom lens according to an aspect of the present inventionsatisfies the relationships:−0.018<(1/f1 /abe1 )/(1/frear)<01.7<nd11<1.79

where f1 is the focal length of the first negative lens as seen from theside having the longer conjugate distance, where abe1 is the Abbe numberand where nd11 is the refractive index at the d line, and where frear isthe synthetic focal length from the second lens group to the fourth lensgroup at the wide angle end.

An eleventh zoom lens according to an aspect of the present inventioncomprises, as seen from the side having the shorter conjugate distance,four lenses, wherein in the order from the side having the longerconjugate distance, a negative meniscus lens whose convex surface facesthe side having the longer conjugate distance, a positive lens, anegative meniscus lens whose convex surface faces the side having theshorter conjugate distance and a positive lens, and the zoom lenssatisfies the relationships:nd4>1.75vd4>401<f4 r/bfw<4

where nd4 is the refractive index at the d line of the negative meniscuslens that is on the side having the longer conjugate distance, where vd4is the Abbe number, where f4r is the focal length of the four lenses andwhere bfw is the air equivalent back focus that does not include a prismand a cover glass when at the wide angle end.

A first wide angle lens according to an aspect of the present inventioncomprises, in the order from the side having the longer conjugatedistance, a first lens group having negative refractive power, a secondlens group, and a third lens group having positive refractive power,wherein the refractive power of the second lens group is weaker than therefractive power of the first lens group and the third lens group,wherein when the magnification is changed from near to far, the firstlens group, the second lens group and the third lens group move alongthe optical axis, wherein when the magnification is changed from near tofar, the air space between the first lens group and the second lensgroup reduces, the air space between the second lens group and the thirdlens group increases, wherein an aperture stop is positioned between thesecond lens group and the third lens group, and wherein the followingrelationship is satisfied:4<bf/f<6

where bf is the air equivalent back focus when the wide angle lens is atinfinity and where f is the focal length of the wide angle lens.

A second wide angle lens according to an aspect of the presentinvention, comprises at least three lens groups, wherein, as seen fromthe side having the longer conjugate distance, the front lens is anegative lens and wherein the following relationships are satisfied:−0.025<(1/f1 /abe1 )/(1/f3 g)<−0.0081.7<nd11<1.79

where f1 is the focal length of the negative lens, where abe1 is theAbbe number and where nd11 is the refractive index at the d line, andwhere f3g is the focal length of the third lens group.

A third wide angle lens according to an aspect of the present inventioncomprises, as seen from the side having the shorter conjugate distance,four lenses, wherein in the order from the side having the longerconjugate distance, a negative meniscus lens whose convex surface facesthe side having the longer conjugate distance, a positive lens, anegative meniscus lens whose convex surface faces the side having theshorter conjugate distance and a positive lens, and wherein thefollowing relationships are satisfied:nd4>1.75vd4>351<f4 r/bf<1.5

where nd4 is the refractive index at the d line of the negative meniscuslens that is on the side having the longer conjugate distance, where vd4is the Abbe number, where f4r is the focal length of the four lenses andwhere bf is the air equivalent back focus that does not include a prismand a cover glass.

A video enlarging/projecting system according to an aspect of thepresent invention comprises a projecting lens in which the above-notedzoom lens or wide angle lens is used, a light source, and a spatialoptical modulating element that is illuminated by the light irradiatedfrom the light source, and that forms an optical image, wherein theprojecting lens projects the optical image that is formed on the spatialoptical modulating element.

A video projector according to an aspect of the present inventioncomprises a projecting lens in which the above-noted zoom lens or wideangle lens is used, a light source, means for temporally restrictinglight from the light source to three colors of blue, green and red, anda spatial optical modulating element that is illuminated by the lightirradiated from the light source, and that forms an optical image thatcorresponds to three colors of blue, green and red that temporallychange.

A rear projector according to an aspect of the present inventioncomprises the above noted video projector, a mirror that bends lightthat is projected from a projecting lens, and a transmissive-type screenfor reflecting an image of projected light.

A multivision system according to an aspect of the present inventioncomprises a plurality of systems that comprise the above noted videoprojector, a transmissive-type screen for reflecting an image ofprojected light, and a casing, and further comprises an image separatingcircuit for separating images.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view of the configuration of the wide angle end of a zoomlens according to Embodiment 1 of the present invention.

FIG. 2 is a view of the configuration of the telephoto end of the zoomlens according to Embodiment 1 of the present invention.

FIG. 3 shows aberration charts for the wide angle end according toWorking Example 1 of the present invention.

FIG. 4 shows aberration charts for the telephoto end according toWorking Example 1 of the present invention.

FIG. 5 is a view of the configuration of the wide angle end of a zoomlens according to Embodiment 2 of the present invention.

FIG. 6 is a view of the configuration of the telephoto end of the zoomlens according to Embodiment 2 of the present invention.

FIG. 7 shows aberration charts for the wide angle end according toWorking Example 2 of the present invention.

FIG. 8 shows aberration charts for the telephoto end according toWorking Example 2 of the present invention.

FIG. 9 is a view of the configuration of the wide angle end of a zoomlens according to a comparative example of the present invention.

FIG. 10 is a view of the configuration of the telephoto end of the zoomlens according to a comparative example of the present invention.

FIG. 11 shows aberration charts for the wide angle end according to acomparative example of the present invention.

FIG. 12 shows aberration charts for the telephoto end according to acomparative example of the present invention.

FIG. 13 is a view of the configuration of the wide angle end of a zoomlens according to Embodiment 3 of the present invention.

FIG. 14 is a view of the configuration of the telephoto end of the zoomlens according to Embodiment 3 of the present invention.

FIG. 15 shows aberration charts for the wide angle end according toWorking Example 3 of the present invention.

FIG. 16 shows aberration charts for the telephoto end according toWorking Example 3 of the present invention.

FIG. 17 is a view of the configuration of the wide angle end of a zoomlens according to Embodiment 4 of the present invention.

FIG. 18 is a view of the configuration of the telephoto end of the zoomlens according to Embodiment 4 of the present invention.

FIG. 19 shows aberration charts for the wide angle end according toWorking Example 4 of the present invention.

FIG. 20 shows aberration charts for the telephoto end according toWorking Example 4 of the present invention.

FIG. 21 is a view of the configuration of the wide angle end of a zoomlens according to Working Example 5 of the present invention.

FIG. 22 is a view of the configuration of the telephoto end of the zoomlens according to Working Example 5 of the present invention.

FIG. 23 shows aberration charts for the wide angle end according toWorking Example 5 of the present invention.

FIG. 24 shows aberration charts for the telephoto end according toWorking Example 5 of the present invention.

FIG. 25 is a view of the configuration of the wide angle end of a zoomlens according to Working Example 6 of the present invention.

FIG. 26 is a view of the configuration of the telephoto end of the zoomlens according to Working Example 6 of the present invention.

FIG. 27 shows aberration charts for the wide angle end according toWorking Example 6 of the present invention.

FIG. 28 shows aberration charts for the telephoto end according toWorking Example 6 of the present invention.

FIG. 29 is a view of the configuration of the wide angle end of a wideangle lens according to Embodiment 5 of the present invention.

FIG. 30 shows aberration charts according to Working Example 7 of thepresent invention.

FIG. 31 is a view of the configuration of a wide angle lens according toWorking Example 8 of the present invention.

FIG. 32 shows aberration charts according to Working Example 8 of thepresent invention.

FIG. 33 is a view of the configuration of a wide angle lens according toWorking Example 9 of the present invention.

FIG. 34 shows aberration charts according to Working Example 9 of thepresent invention.

FIG. 35 is a view of the configuration of a video enlarging/projectingsystem according to Embodiment 6 of the present invention.

FIG. 36 is a view of the configuration of a video projector according toEmbodiment 7 of the present invention.

FIG. 37 is a view of the configuration of a rear projector according toEmbodiment 8 of the present invention.

FIG. 38 is a view of the configuration of a multivision system accordingto Embodiment 9 of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

With the first zoom lens of the present invention, the zoom lens has alens that has positive refractive power on the side having the longerconjugate distance of a second lens group that has negative refractivepower, and thus it is possible to suppress distortion aberration to asmall value.

With the second zoom lens of the present invention, it is possible toreduce lateral chromatic aberration to a small value. frear is the focallength of the lens group on the side having the shorter conjugatedistance, with respect to the aperture stop, and it represents theamount of over-correction of blue lateral chromatic aberration of thelens group on the side having the shorter conjugate distance, withrespect to the aperture stop. f1/abe1 represents the amount of bluelateral chromatic aberration that is generated. It is possible to cancelout the over-correction of the blue lateral chromatic aberration that isgenerated by the lens group on the side having the shorter conjugatedistance, with respect to the aperture stop, with the amount of bluelateral chromatic aberration that is generated by the front lens, asseen from the side having the longer conjugate distance, and to suppresslateral chromatic aberration to a small value. nd11 is the refractiveindex at the d line, of the front lens, as seen from the side having thelonger conjugate distance, and the amount of blue lateral chromaticaberration that is generated increases with increasing refractive index.However, since the blue internal transmittance worsens with increasingrefractive index, the brightness of blue becomes darker.

With the third zoom lens according to the present invention it ispossible to suppress distortion aberration and lateral chromaticaberration to small values. There is significant generation ofdistortion aberration and lateral chromatic aberration in lenses on theside having the shorter conjugate distance, and the refractive power andshape of the lenses are important in the correction of this aberration.The configuration of the four lenses, as seen from the side having thelonger conjugate distance, includes, from the side having the longerconjugate distance, a negative meniscus lens whose convex surface facesthe side having the longer conjugate distance, and is such that it hasgreat ability to correct distortion aberration and lateral chromaticaberration. nd4 and vd4 are the refractive index and Abbe number of thenegative meniscus lens, and they represent conditions for suppressingover-correction of blue lateral chromatic aberration. f4r/bfw representsthe ratio of the focal length of the four lenses from the side havingthe shorter conjugate distance to the air equivalent back focus thatdoes not include prisms or the cover glass at the wide angle end, andrelates to correction of distortion aberration and lateral chromaticaberration, the overall length of the lens, and the outer diameter ofthe lens on the side having the longer conjugate distance.

With the fourth to sixth zoom lenses of the present invention, it ispossible to realize a high definition zoom lens while obtaining a longback focus at a wide angle.

With the seventh to eighth zoom lens of the present invention, it ispossible to provide a compact zoom lens, and whose lens outside diameteris small.

With the ninth zoom lens of the present invention, the first lens grouphas negative power, and since the pupil on the side having the longerconjugate distance is moved toward the side having the longer conjugatedistance, it is possible to make the outside diameter of the first lensgroup small. Furthermore, since the second to fourth lens groups havefavorable aberration correction across the entire region when changingmagnification from the wide angle end to the telephoto end, each aremoved to the side having the longer conjugate distance. The aperturestop is within the second lens group, and this prevents fluctuations inthe position of the pupil on the side having the shorter conjugatedistance. With this configuration, it is possible to realize a compactzoom lens while realizing a long back focus.

With the tenth zoom lens of the present invention, it is possible tosuppress lateral chromatic aberration to a small value. frear is thecombined focal length of the lens groups from the second lens group tothe fourth lens group when at the wide angle end, and represents theamount of over-correction of blue lateral chromatic aberration of thelens groups from the second lens group to the fourth lens group. f1/abe1represents the amount of blue lateral chromatic aberration that isgenerated by the front negative lens, as seen from the side having thelonger conjugate distance. By satisfying the above-noted relationship,it is possible to cancel out the over-correction of blue lateralchromatic aberration that occurs in the lens groups from the second lensgroup to the fourth lens group with the amount of blue lateral chromaticaberration that is generated by the front negative lens, as seen fromthe side having the longer conjugate distance, and to suppress lateralchromatic aberration to a small value. nd11 is the refractive index atthe d line of the front negative lens, as seen from the side having thelonger conjugate distance, and the amount of blue lateral chromaticaberration that is generated increases with increasing refractive index.However, since the blue internal transmittance worsens with increasingrefractive index, the brightness of blue becomes darker. Thus, it ispossible to achieve a balance between the amount of lateral chromaticaberration that is generated and the internal transmittance by theabove-noted relationship.

With the eleventh zoom lens of the present invention, it is possible tosuppress distortion aberration and lateral chromatic aberration to smallvalues. There is significant generation of distortion aberration andlateral chromatic aberration in lenses on the side having the shorterconjugate distance, and refractive power and shape of the lenses areimportant in the correction of this aberration. Thus, two of the fourlenses that are on the side having the shorter conjugate distance arenegative meniscus lenses that have a great ability to correct distortionaberration and lateral chromatic aberration. nd4 and vd4 are therefractive index and Abbe number of the negative meniscus lens and theyrepresent conditions for suppressing over-correction of blue lateralchromatic aberration. f4r/bfw represents the ratio of the focal lengthof the four lenses from the side having the shorter conjugate distanceto the air equivalent back focus that does not include prisms or thecover glass at the wide angle end, and relates to correction ofdistortion aberration and lateral chromatic aberration, the overalllength of the lens, and the outer diameter of the lens on the sidehaving the longer conjugate distance.

With the first wide angle lens of the present invention, it is possibleto realize a wide angle lens whose performance changes little withchanges in the projection distance, while realizing a long back focus.With the second wide angle lens of the present invention, it is possibleto provide lateral chromatic aberration that is small. With the thirdwide angle lens of the present invention, it is possible to suppressdistortion aberration and lateral chromatic aberration to small values.

With the video projecting/enlarging system of the present invention, itis possible to project an image that has little distortion.

With the video projector of the present invention, since the lateralchromatic aberration is favorably corrected, it is possible to projectimages in three colors of blue, green and red onto a screen withoutcolor offset, and to obtain a high definition image.

With the rear projector of the present invention, it is possible torealize a set in which a high definition screen may be obtained.

With the multivision system of the present invention, since distortionsare corrected favorably, the joints between the video projectorscorrespond favorably, and it is possible to obtain a high definitionscreen.

In the first zoom lens, it is preferable that the refractive power ofthe lenses of the second lens group is, as seen from the side having thelonger conjugate distance, positive, negative, negative, positive,negative. It is also preferable that the refractive power of the lensesof the second lens group is, as seen from the side having the longerconjugate distance, positive, negative, negative, negative, positive,negative.

It is possible to suppress distortion aberration and lateral chromaticaberration to small values, and to provide favorable balance with otheraberrations by arranging the refractive power in a similar manner to theabove noted second lens group. It is usual that the second group havingnegative refractive power is constituted by lenses having, from the sidehaving the longer conjugate distance, negative, negative and positiverefractive power. Attaching a lens having positive refractive power tothe side having the longer conjugate distance means increasing therefractive power of the negative lens, and it is possible to realize azoom lens that has little distortion aberration and lateral chromaticaberration by attaching a lens having negative refractive power to theside having the shorter conjugate distance in order to ensure favorableperformance.

Furthermore, it is preferable to satisfy the relationship:−0.6<f2 g/f2 top<−0.15

where f2top is the focal length of a first lens as seen from the sidehaving the longer conjugate distance of the lenses of the second lensgroup and where f2g is the focal length of the second lens group. Withthis configuration, it is possible to suppress distortion aberration andlateral chromatic aberration to small values and to provide favorablebalance with other aberrations.

Furthermore, it is preferable to satisfy the relationship:0.25<frear/f2 top<0.95

where f2top is the focal length of a first lens as seen from the sidehaving the longer conjugate distance of the lenses of the second lensgroup, and where frear is the focal length of the lens group on the sidehaving the shorter conjugate distance, with respect to an aperture stop.With this configuration, it is possible to suppress distortionaberration and lateral chromatic aberration to small values, and toprovide favorable balance with other aberrations. frear is the focallength of the lens group on the side having the shorter conjugatedistance, with respect to an aperture stop, and represents the amount ofdistortion aberration of the lens group on the side having the shorterconjugate distance, with respect to the aperture stop. f2top representsthe amount of distortion aberration that is generated by the second lensgroup. It is possible to suppress the amount of distortion aberration toa small value by balancing the two.

In the fourth zoom lens, it is preferable to satisfy the relationships:0.05<fw/f1 g<0.2−0.9<fw/f2 g<−0.60.5<fw/f3 g<0.7

where f1g is the focal length of the first lens group, where f2g is thefocal length of the second lens group, where f3g is the focal length ofthe third lens group, and where fw is the focal length of the zoom lensat the wide angle end. With this configuration, it is possible torealize a high definition zoom lens while suppressing the overall lenslength and the outside diameter of the lens.

In the fifth zoom lens, it is preferable to satisfy the relationships:0.3<fw/f1 g<0.4−1.6<fw/f2 g<−1.30.7<fw/f3 g<0.9

where f1g is the focal length of the first lens group, where f2g is thefocal length of the second lens group, where f3g is the focal length ofthe third lens group, and where fw is the focal length of the zoom lensat the wide angle end. With this configuration, it is possible torealize a high definition zoom lens while suppressing the overall lenslength and the outside diameter of the lens.

In the sixth zoom lens, it is preferable to satisfy the relationships:0.45<fw/f1 g<0.6−2.0<fw/f2 g<−1.60.9<fw/f3 g<1.3

where f1g is the focal length of the first lens group, where f2g is thefocal length of the second lens group, where f3g is the focal length ofthe third lens group, and where fw is the focal length of the zoom lensat the wide angle end. With this configuration, it is possible torealize a high definition zoom lens while suppressing the overall lenslength and the outside diameter of the lens.

In the fourth to sixth zoom lens, it is preferable that the Abbe numberof all lenses having positive refractive power that are arranged on theside having the shorter conjugate distance with respect to an aperturestop is at least 80. With this configuration, it is possible to realizea zoom lens that has little lateral chromatic aberration.

Furthermore, it is preferable that the Abbe number of all lenses havingnegative refractive power that are arranged on the side having theshorter conjugate distance with respect to an aperture stop is at least35.

With this configuration, it is possible to realize a zoom lens that haslittle lateral chromatic aberration.

In the first zoom lens to the seventh zoom lens, it is preferable thatthe zoom lens is a projecting lens for a projector.

Furthermore, it is preferable that the magnification ratio of the entirelens system is used in a range of −0.00058 times to −0.0188 times.

Furthermore, it is preferable that the F number is 2.5 or 2.4.

Furthermore, it is preferable that the zoom ratio is 1.5, 1.6 or 1.65.

Furthermore, it is preferable that the zoom lens does not have a joinedsurface.

In the ninth zoom lens, it is preferable that the second lens group isconstituted by at least three lenses, and that the first lens, as seenfrom the side having the longer conjugate distance, has negativerefractive power, and that the second lens has positive refractivepower. With this configuration, it is possible to achieve a long backfocus and it is possible to have favorable balance with otheraberrations.

Furthermore, it is preferable that when changing magnification from thewide angle end to the telephoto end, that the second lens group and thefourth lens group move in the same way along the optical axis from theside having the shorter conjugate distance to the side having the longerconjugate distance. With this configuration, it is possible to suppresschanges in telecentricity over the entire magnification region from thewide angle end to the telephoto end and also to correct coma aberrationfavorably corrected, and it is possible to simplify the structure of thelens barrel that holds the lens groups and to provide a zoom lens thathas high optical performance at low cost.

It is preferable that the following relationships are satisfied:−0.45<fw/f1 g<−0.30.01<fw/f2 g<0.30.18<fw/f3 g<0.290.05<fw/f4 g<0.2

where the focal length of the first lens group is f1g, where the focallength of the second lens group is f2g, where the focal length of thethird lens group is f3g, where the focal length of the fourth lens groupis f4g and where the focal length of the above-noted zoom lens at thewide angle end is fw. With this configuration, it is possible to providea zoom lens that has a long back focus while being compact, to suppressdistortion aberration and lateral chromatic aberration to small valuesand to provide favorable balance with other aberrations.

In the ninth to the eleventh zoom lens, it is preferable that the Abbenumber of all lenses having positive refractive power that constitutethe third lens group and the fourth lens group is at least 80. With thisconfiguration, it is possible to realize a zoom lens having smalllateral aberration.

Furthermore, it is preferable that the zoom lens is a projecting lensfor a projector.

Furthermore, it is preferable that the magnification ratio of the entirelens system is used in a range of −0.00058 times to −0.0188 times.

Furthermore, it is preferable that the F number is 2.5.

Furthermore, it is preferable that the zoom ratio is 1.3.

Furthermore, it is preferable that the zoom lens does not have a joinedsurface.

In the first wide angle lens, it is preferable that when changingmagnification from near to far, the first lens group and the third lensgroup move in the same way along the optical axis. With thisconfiguration, it is possible to simplify the construction of the lensbarrel, and to lower the cost.

Furthermore, it is preferable to satisfy the following relationships:−0.4<f/f1 g<−0.15−0.2<f/f2 g<0.050.15<f/f3 g<0.25

where f1g is the focal length of the first lens group, where f2g is thefocal length of the second lens group, where f3g is the focal length ofthe third lens group, and where f is the focal length of the wide anglelens. With this configuration, it is possible to realize a wide anglelens that is compact, and in which distortion aberration and chromaticaberration are favorably corrected.

In the first to the third wide angle lens of the present invention, itis preferable that all the lenses having positive refractive power thatconstitute the third lens group have a refractive index at the d line of1.65 or less. With this configuration, the Petzval Sum can be suppressedto a small value, and it is possible to suppress curvature of the fieldand astigmatic aberration to small values.

Furthermore, it is preferable that the zoom lens is a projecting lensfor a projector.

Furthermore, it is preferable that the magnification ratio of the entirelens system is used in a range of −0.00058 times to −0.0188 times.

Furthermore, it is preferable that the F number is 2.5.

Furthermore, it is preferable that the zoom lens does not have a joinedsurface.

Embodiments of the present invention are described below with referenceto the drawings.

EMBODIMENT 1

FIG. 1 shows a view of a configuration of the wide angle end of a zoomlens according to Embodiment 1 of the present invention. FIG. 2 shows aview of a configuration of the telephoto end of the zoom lens shown inFIG. 1. A zoom lens 10 shown in FIG. 1 is provided with a first lensgroup 11 that has positive refractive power, a second lens group 12 thathas negative refractive power, and a third lens group 13 that haspositive refractive power, in the order as seen from the side with thelonger conjugate distance. Numeral 14 denotes a glass block such as aprism. Numeral 15 denotes an image surface, and in the case of animage-taking system denotes film or CCDs, and in the case of aprojection device, it denotes LCDs, for example, which are spatialmodulating elements. In the figure, the side with the longer conjugatedistance is the side that is opposite the image surface 15.

When changing magnification from the wide angle end (FIG. 1) to thetelephoto end (FIG. 2), the first lens group 11 and the third lens group13 move to the side having the longer conjugate distance, and the secondlens group 12 moves to the side having the shorter conjugate distance.

The first lens group 11, which has positive refractive power, isconfigured by two lenses, a negative lens 11 a and a positive lens 11 bfrom the side having the longer conjugate distance. A glass materialthat has a high refractive index and a small Abbe number is used for thenegative lens 11 a. Since lateral chromatic aberration on the blue siderapidly increases when lateral chromatic aberration is reduced, blueside lateral chromatic aberration is reduced by using a glass materialwith a small Abbe number.

The second lens group 12 having negative refractive power is a variablemagnification lens group. The second lens group 12 is configured by fivelenses, a positive lens 12 a, a negative lens 12 b, a negative lens 12 ca positive lens 12 d and a negative lens 12 e, from the side having thelonger conjugate distance. The positive lens 12 a on the side having thelonger conjugate distance generates positive distortion on the wideangle end side. In particular, it generates higher order distortions.Since the overall lens system has a negative distortion at the wideangle end, the positive distortion of the positive lens 12 a correctsthe negative distortion of the entire lens system and reduces thedistortion at the wide angle end. The positive lens 12 a uses a glassmaterial that has a high refractive index, and a large Abbe number.Accordingly, this reduces blue side lateral chromatic aberration.Distortion aberration is generated at the same time as lateral chromaticaberration and thus lateral chromatic aberration is reduced by usingglass material that has a large Abbe number.

The third lens group 13 having positive refractive power is a variablemagnification lens group. An aperture stop 16 is positioned within thethird lens group 13, and moves with the third lens group 13 whenchanging magnification, thus suppressing fluctuations in telecentricitywhen changing magnification.

The present embodiment realizes a projecting lens that has lowdistortion through a zoom configuration that is constituted by threegroups, wherein the first lens group 11 having positive refractive poweris at the front when viewed from the side having the longer conjugatedistance, and wherein when viewed from the side having the longerconjugate distance, the first lens 12 a of the second lens group 12,which has negative refractive power, is provided with positiverefractive power. This is described more specifically below.

Distortion of the zoom lens is determined by the refractive power of thelens groups, and the distance of each lens group from the aperture stop.Thus, although the refractive power of the lens groups does not changedue to movement of the lens groups when changing magnification, sincethe distance between the lens groups and the aperture stop does change,fluctuations in distortion occur. In this case, distortions may bereduced by providing the lenses with a shape that is advantageous withrespect to distortion, such as one that is concentric with respect tothe aperture stop, however coma aberration and astigmatic aberrationincrease, and a zoom lens having favorable capabilities may not beobtained.

A zoom lens in which a lens group having positive refractive power isprovided at the front when viewed from the side having the longerconjugate distance is a favorable configuration in that the zoommagnification ratio is easily increased giving a bright lens with a lowF number. For example, a four-group zoom lens that contains lens groupswhose refractive power is positive, negative, positive, positive, whenviewed from the side having the longer conjugate distance, and in whichthe second lens group and the third lens group, when viewed from theside having the longer conjugate distance, move along the optical axiswhen changing the magnification from the wide angle end to the telephotoend, has a fourth lens group that is fixed at the conjugate point on theside that has the shorter conjugate distance. Thus, light bundles thatpass through the fourth lens group are constant without changingaberrations generated by the fourth lens group due to changes inmagnification, and it is possible to realize high optical performance.Furthermore, since the position of the principal light ray that passesthrough the fourth lens group also does not change, if used in aprojector, then the pupil of the illumination system and the pupil ofthe projector coincide, and it is possible to realize a projector thatis bright up to the periphery of the image.

Furthermore, for a reverse telephoto-type lens, what is known as a retrofocus-type lens, that has lens groups that have negative and positiverefractive power when viewed from the side having the longer conjugatedistance, significant distortions occur because of the large asymmetryin the refractive power of the lens groups on either side of theaperture stop, however the pupil is advanced because the first lensgroup has negative refractive power, and since the principal light beamthat passes through the first lens group does so at a position close tothe optical axis, the amount of distortion aberration generated at thefirst lens group is small. In this configuration, the distortion of theoverall lens system is suppressed by attaching a positive lens to theside of the negative first lens group that has the longer conjugatedistance to generate higher order distortion aberration.

Thus, the capacity of such a retro focus-type two-group zoom lens tocorrect distortion is high, and such lenses often are used as wide anglezoom lenses. Furthermore, the back focus also may be lengthened.However, it is difficult to lower the F number, the F number changeswhen the magnification is altered with zoom, and it is difficult toobtain a large zoom ratio. Moreover, the back focus changes withmagnification ratio of the zoom, and the position of the pupil on theside having the shorter conjugate distance also moves from the conjugatepoint of the side having the shorter conjugate distance.

By arranging the first lens group 11 that has positive refractive powerat the front, when viewed from the side having the longer conjugatedistance, the F number of the zoom lens 10 shown in FIG. 1 may bereduced and a large zoom ratio is ensured in the same way as with theabove-noted 4-group zoom lens. Furthermore, for the second lens group12, since the positive lens 12 a is attached to the side having thelonger conjugate distance, it is possible to obtain a similar effect tothat of a positive lens attached to the front of the negative lens groupof a retro focus-type lens, such as is noted above, and it is possibleto suppress distortion aberrations to a small value.

That is to say, by arranging the first lens group 11, having positiverefractive power, at the front, when viewed from the side having thelonger conjugate distance, and by attaching the positive lens 12 a onthe side of the second lens group 12 that has the longer conjugatedistance, the F number of the zoom lens according to the presentembodiment can be reduced, a large zoom ratio may be ensured, and it ispossible to suppress distortion aberration to a small value.

Furthermore, the change in the magnification ratio of the second lensgroup 12 with zoom is large, and having a second lens group 12 whoseaberration is independently small is a necessary condition fordemonstrating high optical performance over the entire zoom range. Thesecond lens group 12 of the zoom lens 10 shown in FIG. 1 is constitutedby lenses having positive, negative, negative, positive and negativerefractive power, as viewed from the side having the longer conjugatedistance, and thus the aberration of the second lens group 12 can becorrected down to a small value, and high optical performance may bedemonstrated across the entire zoom range.

A configuration of the zoom lens according to the present invention thatis favorable in terms of optical performance is described below. It ispreferable that the following Expression (1) is satisfied:−0.6<f2 g/f2 top<−0.15  (1)

where f2top is the focal length of the lens 12 a, which is the firstlens 12 a of the second lens group 12 from the side having the longerconjugate distance and where the focal length of the second lens group12 is f2g.

Expression (1) is an expression that prescribes the refractive power ofthe first lens 12 a from the side having the longer conjugate distanceof the second lens group 12 with respect to the overall refractive powerof the second lens group 12. By satisfying expression (1), it ispossible to suppress distortion aberration and lateral chromaticaberration to small values, and to achieve an excellent balance withother aberrations.

If the lower limit of Expression (1) is not met, then the refractivepower of the first lens 12 a of the second lens group 12 increases, theaberration of the second lens group 12 increases, and changes in opticalperformance with zoom also increase. If the upper limit is exceeded,then the refractive power of the first lens 12 a of the second lensgroup 12 becomes small, the distortion correcting effect decreases, andfluctuations in distortion aberration with zoom increase.

Next, it is preferable to satisfy the following Expression (2):0.25<frear/f2 top<0.95  (2)

where f2top is the focal length of the lens 12 a, which is the firstlens of the second lens group 12 from the side having the longerconjugate distance, and frear is the focal length of the lens group onthe side having the shorter conjugate distance with respect to theaperture stop 16 (rear side) (lens 13 c to lens 13 i).

Expression (2) is an expression that prescribes the refractive power ofthe first lens 12 a from the side having the longer conjugate distanceof the second lens group 12, with respect to the refractive power of thelens group that is on the side having the shorter conjugate distancewith respect to the aperture stop 16.

The size of the refractive power of the lens group on the rear side ofthe aperture stop 16 is related to the amount of distortion aberrationthat is generated. The lens that cancels out the distortion aberrationthat is generated in the lens group on the rear side of the aperturestop 16 is the first positive lens 12 a on the side having the longerconjugate distance of the second lens group 12. It is necessary toachieve a balance of the refractive power of both members, andExpression (2) represents that balance. It is possible to suppress thedistortion aberration and the lateral chromatic aberration to a smallvalue and to achieve an excellent balance with other aberrations bysatisfying Expression (2).

If the lower limit of Expression (2) is not met, then the refractivepower of the first lens 12 a of the second lens group 12 decreases, thenthe distortion becomes increasingly negative, and fluctuations of thedistortion with zoom also become large. If the upper limit of Expression(2) is exceeded, then the refractive power of the first lens 12 aincreases, the aberration of the second lens group 12 increases, andchanges in optical performance with zoom become large.

Next, it is preferable to satisfy the following Expression (3) andExpression (4):−0.018<(1/f1 /abe1 )/(1/frear)<0  (3)1.7<nd11<1.79  (4)

where f1 is the focal length of the front negative lens 11 a of thefront lens group (first lens group 11) as seen from the side having thelonger conjugate distance, where abe1 is the Abbe number, where nd11 isthe refractive index of the d line, and where frear is the focal lengthof the lens group on the side having the shorter conjugate distance withrespect to the aperture stop 16 (rear side) (lens 13 c to lens 13 i).

When the chromatic aberration of the lens group on the side having theshorter conjugate distance with respect to the aperture stop 16 iscorrected, blue lateral chromatic aberration is over corrected. The lensthat cancels out the over-corrected blue lateral chromatic aberration isthe front negative lens 11 a, as seen from the side having the longerconjugate distance, of the front lens group 11.

Expression (3) is an expression that represents the relationship betweenthe amount of blue lateral chromatic aberration generated by the frontnegative lens 11 a of the front lens group 11 as seen from the sidehaving the longer conjugate distance, and the amount of over correctionof blue lateral chromatic aberration in the lens group that is on theside having the shorter conjugate distance with respect to the aperturestop 16 If the lower limit of Expression (3) is not met, then correctionof blue lateral chromatic aberration is insufficient and correction ofred lateral chromatic aberration is also insufficient. If the upperlimit is exceeded, then blue lateral chromatic aberration increases dueto over correction.

Furthermore, it is preferable that the negative lens 11 a that is at thefront as seen from the side having the longer conjugate distance has ahigh refractive index and a small Abbe number. However, the glassmaterial as noted above is characterized in that the internaltransmittance deteriorates. Expression (4) is an expression thatprescribes the refractive index of the negative lens at the front of thelens group that is at the front as seen from the side having the longerconjugate distance. When the lower limit is not met, the over-correctionof blue lateral chromatic aberration cannot be decreased, and when theupper limit is exceeded, the internal transmittance decreases and thecolor balance worsens.

Next, four lenses (lenses 13 f, 13 g, 13 h and 13 i) from the sidehaving the shorter conjugate distance are constituted, in order from theside having the longer conjugate distance, by a negative meniscus lens13 f whose convex surface faces the side having the longer conjugatedistance, a positive lens 13 g, a negative meniscus lens 13 h whoseconvex surface faces the side having the shorter conjugate distance anda positive lens 13 i. It is preferable to satisfy Expression (5) toExpression (7) below:nd4>1.75  (5)vd4>40  (6)1<f4 r/bfw<4

where nd4 is the refractive index of the d line of the negative meniscuslens 13 f on the side having the longer conjugate distance, where vd4 isthe Abbe number, where f4r is the focal length of the four lenses (lens13 f, 13 g, 13 h and 13 i) from the side having the shorter conjugatedistance and where bfw is the air equivalent back focus at the wideangle end that does not include prisms or the cover glass.

Since the convex surfaces of the negative meniscus lenses 13 f and 13 hface different directions, they may be advantageously applied to reducelateral chromatic aberration and distortion aberration. The negativemeniscus lens 13 f whose convex surface faces the end having the longerconjugate distance is used effectively to correct distortion aberration,and the negative meniscus lens 13 h whose convex surface faces the sidehaving the shorter conjugate distance is used effectively to correctlateral chromatic aberration.

Expression (5) represents the refractive index, at the d line, of thenegative meniscus lens 13 f that is on the side that has the longerconjugate distance, and when the lower limit is not met, the curvatureof the field increases. Expression (6) represents the Abbe number of thenegative meniscus lens 13 f that is on the side having the longerconjugate distance, and when the lower limit is not met, the lateralchromatic aberration increases.

Expression (7) indicates that the focal length of the four lenses, fromthe end having the shorter conjugate distance is larger than the airequivalent back focus that does not include the prism or the cover glassat the wide angle end. That is to say, it shows the use of the lens in amanner in which when an F number light beam is incident on the fourlenses from the side having the shorter conjugate distance the lightconverges toward the side having the shorter conjugate distance. Whenthe lower limit is not met, the external diameter of the lens on theside having the longer conjugate distance increases, and distortionaberration and lateral chromatic aberration increase. When the upperlimit is exceeded, the overall length of the lens increases, and it maybe impossible to ensure the back focus.

Next, when the magnification changes from wide angle to telephoto, thefirst lens group 11, the second lens group 12 and the third lens group13 move along the optical axis. The first lens group 11 movesmonotonically toward the side having the longer conjugate distance, thesecond lens group 12 moves monotonically toward the side having theshorter conjugate distance and the third lens group 13 movesmonotonically toward the side having the longer conjugate distance. Itis preferable to satisfy Expression (8) below, and the relationship ofExpression (8a) also may be satisfied:0.5<bfw/fw<2.4  (8)1.6<bfw/fw<2.4  (8a)

where bfw is the air equivalent back focus when the zoom lens 10 at thewide angle end at infinity, and where fw is the focal length of the zoomlens 10 at the wide angle end.

Expression (8) prescribes the back focus. If the projecting lens of aprojector is not met, and in particular if reflective-type spatialmodulation elements are used, then a long back focus is necessary. Whenthe lower limit is not met, the arrangement of prisms that are insertedbetween the lens and the spatial modulating elements becomes complex,and configuration of the system may not be possible. It is notpreferable to exceed the upper limit because the overall length of thelens and the outer diameter become large.

Next, it is preferable to satisfy the following Expressions (9) to (11),and Expressions (9a) to (11a) may also be satisfied:0.05<fw/f1 g<0.6  (9)−2<fw/f2 g<−0.6  (10)0.5<fw/f3 g<1.3  (11)0.05<fw/f1 g<0.2  (9a)−0.9<fw/f2 g<−0.6  (10a)0.5<fw/f3 g<0.7  (11a)

where f1g is the focal length of the first lens group 11, where f2g isthe focal length of the second lens group 12, where f3g is the focallength of the third lens group 13 and where fw is the focal length ofthe zoom lens 10 at the wide angle end.

Expression (9) prescribes the focal length of the first lens group 11.When the lower limit is not met, the amount of magnification change withzoom is reduced. When the upper limit is exceeded, it is not possible toensure the back focus. Expression (10) is an expression that prescribesthe focal length of the second lens group 12. When the lower limit isnot met, fluctuations of the coma aberration at the wide angle end andthe telephoto end become large. When the upper limit is exceeded, inaddition to not being able to ensure the back focus, the amount that thesecond lens group moves increases, and the size of the lens becomeslarge.

Expression (11) prescribes the focal length of the third lens group 13.When the lower limit is not met, it is not possible to ensure thetelecentricity, and the overall length of the lens increases. When theupper limit is exceeded, it is not possible to ensure thetelecentricity, and the outside diameter of the lens increases.

Next, it is preferable that all the lenses that have positive refractivepower that are positioned on the side that has the shorter conjugationdistance with respect to the aperture stop 16 (lenses 13 c, 13 d, 13 gand 13 i) are configured with an Abbe number of at least 80. The totalrefractive power of the lens group on the side that has the shorterconjugate distance with respect to the aperture stop 16 is positive, theprincipal light beam is significantly bent in order to ensuretelecentricity, and there is significant chromatic aberration. Lateralchromatic aberration is particularly large. If all the lenses havingpositive refractive power that are positioned on the side of theaperture stop 16 that has the shorter conjugate distance are constitutedwith an Abbe number of at least 80, then it is possible to reducelateral chromatic aberration to a small value.

Next, it is preferable that all the lenses that have negative refractivepower that are positioned on the side of the aperture stop 16 that hasthe shorter conjugation distance (lenses 13 e, 13 f and 13 h) areconfigured with an Abbe number of at least 35. The total refractivepower of the lens group on the side of the aperture stop 16 that has theshorter conjugate distance is positive, the principal light beam issignificantly bent in order to ensure telecentricity, and there issignificant chromatic aberration. Lateral chromatic aberrationparticularly increases. It is possible to reduce lateral chromaticaberration to a small value by configuring all the lenses havingnegative refractive power that are positioned on the side of theaperture stop 16 that has the shorter conjugate distance, with materialhaving an Abbe number of at least 35.

Next, when the magnification changes from wide angle to telephoto, thefirst lens group 11, the second lens group 12 and the third lens group13 move along the optical axis. The first lens group 11 movesmonotonically toward the side having the longer conjugate distance, thesecond lens group 12 moves monotonically toward the side having theshorter conjugate distance, the third lens group 13 moves monotonicallytoward the side having the longer conjugate distance and the aperturestop 16 moves with the third lens group 13. It is preferable to satisfythe following Expression (12):|(DG1 −DG3 )/fw|<0.15  (12)

where DG1 is the amount that the first lens group 11 moves from the wideangle end to the telephoto end, where DG3 is the amount that the thirdlens group 13 moves from the wide angle end to the telephoto end, andwhere fw is the focal length of the zoom lens 10 at the wide angle end.

Expression (12) is an expression that prescribes the difference in theamount of movement of the first lens group 11 and the third lens group13, and when this range is exceeded, the outer diameter of the lensesbecome larger.

WORKING EXAMPLE 1

Working Example 1 according to Embodiment 1 is described below. The lensconfiguration of Working Example 1 is the same as the configuration inFIG. 1 and FIG. 2, and is a design example in which F_(NO)=2.5 at thewide angle end, focal length f=37.08 and a half angle of view is 24.2°.The values of the above noted Expressions (1) to (12) in Working Example1 are as follows:f2 g/f2 top=−0.22  Expression (1)frear/f2 top=0.33  Expression (2)(1/f1 /abe1 )/(1/frear)=−0.01  Expression (3)nd11=1.78472  Expression (4)nd4=1.7725  Expression (5)vd4=49.62  Expression (6)f4 r/bfw=1.08  Expression (7)bfw/fw=2.07  Expression (8)fw/f1 g=0.16  Expression (9)fw/f2 g=−0.78  Expression (10)fw/f3 g=0.59  Expression (11)|(DG1 −DG3 )/fw|=0.11  Expression (12)

Next, specific figures are shown in Table 1, and zoom data is shown inTable 2. In Table 1, ri (mm) is the radius of curvature of the lenssurface, di (mm) is the lens thickness or the interval between lenses,ni is the refractive index of the lens at the d line, and vi is the Abbenumber of the lens at the d line. This is the same in Tables 3, 5 and 7below. In the example of Table 1, r1 to r4 is the first lens group, r5to r14 is the second lens group, r15 to r33 is the third lens group, andr19 is the aperture stop.

TABLE 1 ri di Nd νd r1 = 106.344 d1 = 3.4 n1 = 1.78472 v1 = 25.72 r2 =72.165 d2 = 0.8 r3 = 76.842 d3 = 11.3 n2 = 1.62299 v2 = 58.12 r4 =1533.061 d4 = Variable r5 = 160.815 d5 = 4.8 n3 = 1.77250 v3 = 49.62 r6= 2545.794 d6 = 0.8 r7 = 114.985 d7 = 2.5 n4 = 1.49700 v4 = 81.61 r8 =26.964 d8 = 10.8 r9 = −197.147 d9 = 2.1 n5 = 1.49700 v5 = 81.61 r10 =35.249 d10 = 1.6 r11 = 34.148 d11 = 6.2 n6 = 1.74950 v6 = 35.04 r12 =83.884 d12 = 5.8 r13 = −40.964 d13 = 2.0 n7 = 1.80420 v7 = 46.50 r14 =−66.146 d14 = Variable r15 = 195.658 d15 = 2.0 n8 = 1.88300 v8 = 40.80r16 = 54.286 d16 = 14.2 r17 = 115.173 d17 = 4.0 n9 = 1.59270 v9 = 35.45r18 = −132.468 d18 = 0.0 r19 = 0.000 d19 = 33.7 r20 = 192.792 d20 = 7.5n10 = 1.49700 v10 = 81.61 r21 = −91.057 d21 = 0.6 r22 = 44.779 d22 = 8.9n11 = 1.49700 v11 = 81.61 r23 = 189.368 d23 = 0.4 r24 = 225.543 d24 =2.6 n12 = 1.80420 v12 = 46.50 r25 = 50.710 d25 = 14.2 r26 = 179.268 d26= 4.7 n13 = 1.77250 v13 = 49.62 r27 = 59.193 d27 = 1.9 r28 = 83.147 d28= 12.0 n14 = 1.45650 v14 = 90.27 r29 = −51.009 d29 = 2.5 r30 = −45.165d30 = 3.5 n15 = 1.83400 v15 = 37.34 r31 = −63.789 d31 = 0.3 r32 = 77.189d32 = 12.2 n16 = 1.45650 v16 = 90.27 r33 = −90.597 d33 = Variable r34 =0.000 d34 = 88.0 n17 = 1.51680 v17 = 64.20

TABLE 2 Wide angle end Telephoto end d4 2.0 25.97 d14 30.0 2.0 d33 19.3828.0

The charts in FIG. 3 show the spherical aberration (mm), astigmaticaberration (mm) and distortion aberration (%) at the wide angle end inWorking Example 1, and these are the same as in FIG. 7, FIG. 11 and FIG.15 below. The charts in FIG. 4 show the spherical aberration (mm),astigmatic aberration (mm) and distortion aberration (%) at thetelephoto end of Working Example 1, and these are the same as in FIG. 8,FIG. 12 and FIG. 16 below. As can be seen in FIG. 3 and FIG. 4, the zoomlens according to Working Example 1 shows favorable performance withrespect to aberration.

EMBODIMENT 2

FIG. 5 shows a view of a configuration of the wide angle end of a zoomlens according to Embodiment 2 of the present invention. FIG. 6 shows aview of a configuration of the telephoto end of the zoom lens shown inFIG. 5. A zoom lens 20 shown in FIG. 5 is provided with a first lensgroup 21 that has positive refractive power, a second lens group 22 thathas negative refractive power, and a third lens group 23 that haspositive refractive power, in the order as seen from the side with thelonger conjugate distance. The glass block 14, the image surface 15 andthe aperture stop 16 are configured in the same way as in FIG. 1 and areprovided with the same symbols, so their description is hereby omitted.In this drawing also, the side having the longer conjugate distance isthe side opposite the image surface 15.

When changing magnification from the wide angle end (FIG. 5) to thetelephoto end (FIG. 6), the first lens group 21 and the third lens group23 move to the side having the longer conjugate distance, and the secondlens group 22 moves to the side having the shorter conjugate distance.

The first lens group 21, which has positive refractive power, isconfigured by two lenses, a negative lens 21 a and a positive lens 21 bfrom the side having the longer conjugate distance. The negative lensuses a glass material that has a high refractive index and a small Abbenumber. Since the blue side lateral chromatic aberration rapidlyincreases when the lateral chromatic aberration is reduced, the lateralchromatic aberration on the blue side is reduced by using a glassmaterial with a small Abbe number.

The second lens group 22, which has negative refractive power, is avariable magnification lens group. The second lens group 22 isconstituted by five lenses, a positive lens 22 a, a negative lens 22 b,a negative lens 22 c a positive lens 22 d and a negative lens 22 e fromthe side having the longer conjugate distance. The positive lens 22 a onthe side having the longer conjugate distance generates positivedistortion on the wide angle end side. In particular, this generateshigher order distortions. Since the overall lens system is negativelydistorted at the wide angle end, the positive distortion of the positivelens 22 a corrects the overall negative distortion of the optical systemand reduces the distortion at the wide angle end. The positive lens 22 auses a glass material that has a high refractive index, and a large Abbenumber. Accordingly, this reduces blue side chromatic aberration.Distortion aberration is generated at the same time as the lateralchromatic aberration, and thus lateral chromatic aberration is reducedby using glass material that has a large Abbe number.

The third lens group 23 (lens 23 a to lens 23 h), which has positiverefractive power, is a variable magnification lens group. An aperturestop 16 is positioned within the third lens group 23, and moves with thethird lens group 23 when changing magnification, thus suppressingfluctuations in telecentricity when changing magnification.

Also the present embodiment preferably satisfies the above-notedexpressions. It is also preferable to substitute Expression (8) andsatisfy Expression (8b) below:1<bfw/fw<1.8.  (8b)

As a substitute for the above noted Expressions (9) to (11), it ispreferable to satisfy Expressions (9b) to (11b), and to satisfy theabove-noted Expression (12)0.3<fw/f1 g<0.4  (9b)−1.6<fw/f2 g<−1.3  (10b)0.7<fw/f3 g<0.9.  (11b)

WORKING EXAMPLE 2

Working Example 2 according to Embodiment 2 is described below. The lensconfiguration of Working Example 2 is the same as the configuration inFIG. 5 and FIG. 6, and is a design example in which F_(NO)=2.4 at thewide angle end, focal length f=55.83 and a half angle of view is 16.7°.The values of the above noted Expressions (1) to (7), (8b) to (11b) and(12) in Working Example 2 are as follows:f2 g/f2 top=−0.15  Expression (1)frear/f2 top=0.276  Expression (2)(1/f1 /abe1 )/(1/frear)=−0.0144  Expression (3)nd11=1.78472  Expression (4)nd4=1.7725  Expression (5)vd4=49.62  Expression (6)f4 r/bfw=1.67  Expression (7)bfw/fw=1.39  Expression (8b)fw/f1 g=0.394  Expression (9b)fw/f2 g=−1.5  Expression (10b)fw/f3 g=0.82  Expression (11b)|(DG1 −DG3 )/fw|=7.5×10⁻⁵  .Expression (12)

Next, specific figures are shown in Table 3, and zoom data is shown inTable 4. In Table 3, r1 to r4 is the first lens group, r5 to r14 is thesecond lens group, r15 to r31 is the third lens group, and r15 is theaperture stop.

TABLE 3 ri di Nd νd r1 = 85.995 d1 = 2.9 n1 = 1.78472 v1 = 25.72 r2 =53.333 d2 = 2.6 r3 = 55.903 d3 = 10.7 n2 = 1.69680 v2 = 55.46 r4 =24386.946 d4 = Variable r5 = 201.376 d5 = 5.2 n3 = 1.77250 v3 = 49.62 r6= −4186.792 d6 = 1.0 r7 = 95.797 d7 = 2.2 n4 = 1.49700 v4 = 81.61 r8 =29.920 d8 = 14.4 r9 = −77.957 d9 = 2.5 n5 = 1.77250 v5 = 49.62 r10 =87.234 d10 = 1.7 r11 = 58.021 d11 = 5.9 n6 = 1.84666 v6 = 23.78 r12 =−343.528 d12 = 1.7 r13 = −55.209 d13 = 2.4 n7 = 1.83500 v7 = 42.98 r14 =916.120 d14 = Variable r15 = 0.000 d15 = 18.2 r16 = −29.443 d16 = 11.2n8 = 1.51680 v8 = 64.20 r17 = −37.176 d17 = 1.5 r18 = 62.585 d18 = 12.8n9 = 1.49700 v9 = 81.61 r19 = −77.869 d19 = 0.6 r20 = 73.217 d20 = 8.2n10 = 1.49700 v10 = 81.61 r21 = −228.864 d21 = 3.4 r22 = −75.455 d22 =3.0 n11 = 1.80420 v11 = 46.50 r23 = 553.971 d23 = 16.5 r24 = 106.906 d24= 3.5 n12 = 1.77250 v12 = 49.62 r25 = 47.664 d25 = 4.1 r26 = 144.895 d26= 10.5 n13 = 1.45650 v13 = 90.27 r27 = −68.374 d27 = 2.5 r28 = −51.129d28 = 3.5 n14 = 1.83400 v14 = 37.34 r29 = −72.664 d29 = 0.3 r30 = 66.746d30 = 12.2 n15 = 1.45650 v15 = 90.27 r31 = −139.985 d31 = Variable r32 =0.000 d32 = 88.0 n16 = 1.51680 v16 = 64.17

TABLE 4 Wide angle end Telephoto end d4 3.6 24.6284 d14 28.32 7.2592 d3110.0 20.9021

The charts in FIG. 7 and FIG. 8 show the distortion performances ofWorking Example 2, and the zoom lens according to Working Example 2shows favorable performance with respect to aberration.

COMPARATIVE EXAMPLE 1

FIG. 9 shows a view of a configuration of the wide angle end of a zoomlens according to Comparative Example 1. FIG. 10 shows a view of aconfiguration of the telephoto end of the zoom lens shown in FIG. 9. Azoom lens 100 shown in FIG. 9 is provided with a first lens group 101(101 a and 101 b) that has positive refractive power, a second lensgroup 102 (102 a to 102 c) that has negative refractive power, and athird lens group 103 (103 a to 103 h) that has positive refractivepower, in the order as seen from the side with the longer conjugatedistance. The glass block 14, the image surface 15 and the aperture stop16 are configured the same as in FIG. 5, and the side having the longerconjugate distance is also the same as in FIG. 5.

In Working Example 1 shown in FIG. 5, the second lens group 22 isconstituted by five lenses, having positive, negative, negative positiveand negative refractive power, from the side having the longer conjugatedistance, while in Comparative Example 1 shown in FIG. 9 the second lensgroup 102 is constituted by three lenses, having negative, negative andpositive refractive power. That is to say that while the front lens 22 aof the second lens group 22 of Working Example 1, as seen from the sidehaving the longer conjugate distance has positive refractive power, theequivalent lens 102 a in Comparative Example 1 has negative refractivepower.

Comparative Example 1 is a design example having F_(NO)=2.5 at the wideangle end, focal length f=55.87 and a half angle of view of 16.7°.Specific figures are shown below in Table 5, and zoom data is shown inTable 6. In Table 5, r1 to r4 is the first lens group, r5 to r10 is thesecond lens group, r11 to r27 is the third lens group, and r11 is theaperture stop.

TABLE 5 ri di Nd νd r1 = 95.131 d1 = 3.0 n1 = 1.78472 v1 = 25.72 r2 =56.899 d2 = 2.0 r3 = 63.005 d3 = 10.9 n2 = 1.69680 v2 = 55.46 r4 =−338.163 d4 = Variable r5 = 796.702 d5 = 1.6 n3 = 1.49700 v3 = 81.61 r6= 29.716 d6 = 9.8 r7 = −45.343 d7 = 1.3 n4 = 1.49700 v4 = 81.61 r8 =72.031 d8 = 0.2 r9 = 54.448 d9 = 2.5 n5 = 1.74950 v5 = 35.30 r10 =390.826 d10 = Variable r11 = 0.000 d11 = 17.7 r12 = −28.802 d12 = 11.9n6 = 1.51680 v6 = 64.20 r13 = −36.578 d13 = 1.6 r14 = 66.302 d14 = 13.5n7 = 1.49700 v7 = 81.61 r15 = −79.659 d15 = 0.6 r16 = 73.506 d16 = 8.6n8 = 1.49700 v8 = 81.61 r17 = −3758.630 d17 = 3.6 r18 = −80.693 d18 =3.2 n9 = 1.80420 v9 = 46.50 r19 = −2013.269 d19 = 17.6 r20 = 102.089 d20= 3.7 n10 = 1.77250 v10 = 49.62 r21 = 50.320 d21 = 4.3 r22 = 114.274 d22= 11.1 n11 = 1.45650 v11 = 90.27 r23 = −57.604 d23 = 2.6 r24 = −46.139d24 = 3.7 n12 = 1.83400 v12 = 37.34 r25 = −78.213 d25 = 0.3 r26 =108.833 d26 = 12.9 n13 = 1.45650 v13 = 90.27 r27 = −87.624 d27 =Variable r28 = 0.000 d28 = 88.0 n14 = 1.51680 v14 = 64.17

TABLE 6 Wide angle end Telephoto end d4 6.7 30.55 d10 26.63 3.83 d2710.56 16.29

FIG. 11 shows aberration charts of the wide angle end of the comparativeexample. FIG. 12 shows aberration charts of the wide angle end of thecomparative example. Apart from the second lens group, ComparativeExample 1 has substantially the same configuration as Working Example 2,and the distortion at the wide angle end is −2.4% and is +1% at thetelephoto end. If the aberration charts are compared with those ofWorking Example 2 in FIG. 7 and FIG. 8, it can be seen that thedistortion aberration of Working Example 2 is reduced by adding apositive lens onto the side of the second lens group that has the longerconjugate distance.

EMBODIMENT 3

FIG. 13 shows a view of a configuration of the wide angle end of a zoomlens according to Embodiment 3 of the present invention. FIG. 14 shows aview of a configuration of the telephoto end of the zoom lens shown inFIG. 13. A zoom lens 30 shown in FIG. 13 is provided with a first lensgroup 31 that has positive refractive power, a second lens group 32 thathas negative refractive power, and a third lens group 33 that haspositive refractive power, in the order as seen from the side with thelonger conjugate distance. The glass block 14, the image surface 15 andthe aperture stop 16 are configured the same as in FIG. 1 and areprovided with the same symbols, so their description is hereby omitted.In this drawing also, the side having the longer conjugate distance isthe side opposite the image surface 15.

When changing magnification from the wide angle end (FIG. 13) to thetelephoto end (FIG. 14), the first lens group 31 is fixed, the thirdlens group 33 moves to the side having the longer conjugate distance,and the second lens group 32 moves to the side having the shorterconjugate distance.

The first lens group 31 (lenses 31 a to 31 d) is constituted by fourlenses, that is to say negative, positive, positive and negative lensesas seen from the side having the longer conjugate distance. The secondlens group (lenses 32 a to 32 f) has negative refractive power, and is avariable magnification lens group. The second lens group 32 isconstituted by five lenses, that is to say positive, negative, negative,negative, positive and negative lenses as seen from the side having thelonger conjugate distance. The positive lens 32 a on the side having thelonger conjugate distance generates positive distortion at the wideangle end side. It particularly generates high order distortions. Sincethe wide angle end of the entire lens system has negative distortion,the positive distortion of the positive lens 32 a corrects the negativedistortion of the entire lens system, and reduces the distortion at thewide angle end.

The positive lens 32 is made of a glass material that has a highrefractive index and a large Abbe number. Thus, blue side lateralchromatic aberration may be reduced. Because the lens generatesdistortion aberration at the same time as lateral chromatic aberration,the lateral chromatic aberration is reduced by using a glass materialthat has a large Abbe number.

The third lens group 33 (lenses 33 a to 33 i) has positive refractivepower, and is a variable magnification lens group. The aperture stop 16is positioned between the second lens group 32 and the third lens group33, it moves together with the third lens group 33 when changingmagnification, and it suppresses fluctuations in telecentricity when themagnification changes.

The present embodiment preferably satisfies at least any one of theabove-noted Expressions (1), (2), (5), (6) and (7). Furthermore, as asubstitute for the above-noted Expression (8), it is preferable tosatisfy Expression (8c) below:0.5<bfw/fw<1.3.  (8c)

As a substitute for the above noted Expressions (9) to (11), it ispreferable to satisfy Expression (9c) to (11c) below:0.45<fw/f1 g<0.6  (9c)−2.0<fw/f2 g<−1.6  (10c)0.9<fw/f3 g<1.3.  (11c)

Furthermore, as a substitute for the above-noted Expression (12), it ispreferable to satisfy Expression (12a) below:|DG3/ fw|<0.15.  (12a)

WORKING EXAMPLE 3

Working Example 3 according to Embodiment 3 is described below. The lensconfiguration of Working Example 3 is the same as the configuration inFIG. 13 and FIG. 14, and is a design example in which F_(NO)=2.5 at thewide angle end, focal length f=96.39 and a half angle of view is 10.25°.The values of the above noted Expressions (1), (2), (5) to (7), (8c) to(11c) and (12a) in Working Example 3 are as follows:f2 g/f2 top=−0.54  Expression (1)frear/f2 top=0.86  Expression (2)nd4=1.7725  Expression (5)vd4=49.62  Expression (6)f4 r/bfw=2.05  Expression (7)bfw/fw=0.89  Expression (8c)fw/f1 g=0.53  Expression (9c)fw/f2 g=−1.82  Expression (10c)fw/f3 g=1.14  Expression (11c)|(DG1 −DG3 )/fw|=0.15.  Expression (12a)

Next, specific figures are shown in Table 7, and zoom data is shown inTable 8. In Table 7, r1 to r8 is the first lens group, r9 to r20 is thesecond lens group, r21 to r39 is the third lens group, and r21 is theaperture stop.

TABLE 7 ri di Nd νd r1 = −168.281 d1 = 3.0 n1 = 1.58913 v1 = 61.25 r2 =−250.323 d2 = 1.2 r3 = 182.115 d3 = 8.5 n2 = 1.49700 v2 = 81.61 r4 =−182.115 d4 = 1.0 r5 = 76.767 d5 = 8.9 n3 = 1.77250 v3 = 49.62 r6 =1495.473 d6 = 2.0 r7 = −1659.724 d7 = 3.0 n4 = 1.80610 v4 = 33.27 r8 =88.570 d8 = Variable r9 = 86.983 d9 = 5.7 n5 = 1.77250 v5 = 49.62 r10 =−596.427 d10 = 1.1 r11 = −281.532 d11 = 2.6 n6 = 1.48749 v6 = 70.44 r12= 37.072 d12 = 14.3 r13 = −118.479 d13 = 2.0 n7 = 1.48749 v7 = 70.44 r14= 86.916 d14 = 6.5 r15 = −65.186 d15 = 2.8 n8 = 1.48749 v8 = 70.44 r16 =607.838 d16 = 3.4 r17 = 125.821 d17 = 6.2 n9 = 1.59270 v9 = 35.45 r18 =−134.162 d18 = 2.1 r19 = −96.116 d19 = 2.6 n10 = 1.58913 v10 = 61.25 r20= −1236.869 d20 = Variable r21 = 0.000 d21 = 3.0 r22 = −202.574 d22 =10.9 n11 = 1.4560 v11 = 90.27 r23 = −40.888 d23 = 0.8 r24 = −40.269 d24= 2.3 n12 = 1.58913 v12 = 61.25 r25 = −92.130 d25 = 5.1 r26 = 129.693d26 = 10.0 n13 = 1.45650 v13 = 90.27 r27 = −129.693 d27 = 0.5 r28 =129.693 d28 = 10.0 n14 = 1.45650 v14 = 90.27 r29 = −129.693 d29 = 4.0r30 = −101.932 d30 = 3.0 n15 = 1.58913 v15 = 61.25 r31 = −674.891 d31 =27.0 r32 = 178.904 d32 = 4.0 n16 = 1.77250 v16 = 49.62 r33 = 85.847 d33= 5.0 r34 = 927.613 d34 = 11.5 n17 = 1.49700 v17 = 81.61 r35 = −59.130d35 = 5.5 r36 = −55.922 d36 = 3.0 n18 = 1.83400 v18 = 37.34 r37 =−82.040 d37 = 0.3 r38 = 91.980 d38 = 8.9 n19 = 1.49700 v19 = 81.61 r39 =0.000 d39 = Variable r40 = 0.000 d40 = 93.5 n20 = 1.51680 v20 = 64.17

TABLE 8 Wide angle end Telephoto end d8 6.9 24.7 d20 34.15 2.33 d39 11.025.03

The charts in FIG. 15 and FIG. 16 show the distortion performances ofWorking Example 3, and the zoom lens according to Working Example 3shows favorable performance with respect to aberration.

It should be noted that Embodiments 1 to 3 were described by examples inwhich there were three lens groups, however the lens may be configuredby four or more groups, and provided that the front lens of the secondgroup, as seen from the side having the longer conjugate distance, haspositive refractive power, the effect of the present invention may beobtained.

Furthermore, the configuration for satisfying the above-noted Expression(8), Expressions (9) to (11) and Expression (12) was described suchthat, as in Embodiments 1 to 3, the zoom lens is configured with 3groups wherein it is a prerequisite that the front lens of the secondgroup has positive refractive power, however there is no limitation tothis configuration. That is to say, the effects of satisfying Expression(8), Expressions (9) to (11) as were noted above may be obtained byeither applying such expressions to a zoom lens having four groups ormore, or to a configuration in which the front lens of the second grouphas negative refractive power. Furthermore, even if Expression (12) wereapplied to a configuration in which the front lens of the second grouphas negative refractive power, the effect of satisfying an expressionsuch as Expression (12) may be obtained. This is the same withExpressions (8a) to (8c), Expressions (9a) to (11a), Expressions (9b) to(11b), Expressions (9c) to (11c) and Expression (12a).

EMBODIMENT 4

FIG. 17 is a view of a configuration of the wide angle end of a zoomlens according to Embodiment 4 of the present invention. FIG. 18 is aview of a configuration of the telephoto end of the zoom lens shown inFIG. 17. A zoom lens 110 shown in FIG. 17 is provided with a first lensgroup 111 that has negative refractive power, a second lens group 112that has positive refractive power, and a third lens group 113 that haspositive refractive power, and a fourth lens group 114 that has positiverefractive power, in the order as seen from the side with the longerconjugate distance. Numeral 15 denotes a glass block such as a prism.Numeral 16 denotes an image surface, and in the case of an image-takingsystem denotes film or CCDs, and in the case of a projection device, itdenotes LCDs, for example, which are spatial modulating elements. Itshould be noted that in the example in FIG. 17, the side with the longerconjugate distance is the side that is opposite the image surface 16(the same as in the drawings below).

Furthermore, when changing magnification from the wide angle end (FIG.17) to the telephoto end (FIG. 18), the first lens group 111 moves tothe side having the shorter conjugate distance, the second lens group112 moves to the side having the longer conjugate distance, the thirdlens group 113 moves to the side having the longer conjugate distanceand the fourth lens group 114 moves to the side having the longerconjugate distance.

The first lens group 111 is configured by six lenses, a negative lens111 a, a positive lens 111 b, a negative lens 111 c, a negative lens 111d, a negative lens 111 e and a positive lens 111 f, in that order fromthe side having the longer conjugate distance.

The second lens group 112 is a variable magnification lens group. Thesecond lens group 112 is configured by three lenses, a negative lens 112a, a positive lens 112 b, and a positive lens 112 c, from the sidehaving the longer conjugate distance. In order to ensure the back focusof the entire zoom lens, the second lens group 112 is configured as areverse telephoto-type lens.

The third lens group 113 has a relatively large positive refractiveindex, and has the effect of reducing the load on the fourth lens groupand of moving the position of the aperture stop 17 toward the sidehaving the longer conjugate distance. In order to suppress fluctuationsin aberration due to the change in magnification between the wide angleend to the telephoto end, the third lens group 113 moves slightlydifferently from the second lens group 112 and the fourth lens group114. That is to say, the third lens group 113 does not move as a singlebody with the second lens group 112 and the fourth lens group 114, andthe amount that the third lens group moves also differs from these lensgroups.

The fourth lens group 114 is a variable magnification lens group. Whenchanging magnification, the fourth lens group 114 moves as one with thesecond lens group 112, and is set so as to suppress changes intelecentricity with changes in magnification. The fourth lens group 114greatly influences the distortion and lateral chromatic aberration, andthus is configured to effectively suppress these aberrations. That is tosay, the fourth lens group 114 is constituted by arrangement of aconcave meniscus lens 114 a whose convex surface faces the side havingthe longer conjugate distance, a positive lens 114 b, a concave meniscuslens 114 c whose convex surface faces the side having the shorterconjugate distance, and a positive lens 114 d, in that order from theside having the longer conjugate distance.

As noted above, the present invention ensures the back focus andrealizes a compact zoom lens by a zoom configuration of four groups,having negative, positive, positive and positive refractive power, asseen from the side having the longer conjugate distance. The zoom lens110 is described more specifically below. The zoom lens 110 is basicallya two-group zoom that is constituted by a negative and a positive lensgroup. Two-group zoom is suited to wide angles, and has thecharacteristic in that a long back focus is easily obtained. However,long zoom and large diameters are difficult, and the back focus and theF number changes when the magnification is changed from the wide angleend to the telephoto end.

Regarding what is known as a front positive power-type zoom lens havinga four-group zoom of positive, negative, positive, and positive, orpositive, negative, negative and positive, or a three-group zoom havingpositive, negative and positive refractive power, since the pupil on theside having the longer conjugate distance is positioned on the sidehaving the shorter conjugate distance, the outer diameter of thepositive first lens group increases when it is changed to wide angle.Moreover, in order to ensure that telecentricity does not change withchanges in magnification, in all the configurations it is necessary toposition the aperture stop within the positive lens group that has theshortest conjugate distance, and it is difficult to suppressfluctuations in distortion due to changes in magnification.

In order to use the lens as the projecting lens of a projector, it isnecessary that both distortion and lateral chromatic aberration aresmall to obtain high picture quality, and compactness is desired tofacilitate installation.

Accordingly, in the present embodiment, the external diameter of thenegative first lens group is suppressed to a compact size by forming thezoom lens in a negative power front-type zoom configuration, andpositioning the pupil on the side having the longer conjugate distance.Also, by positioning the position of the aperture stop on the sidehaving the longer conjugate distance, the pupil may be moved further inthe direction of the side having the longer conjugate distance, and thusit is possible to make the outside diameter of the negative first lensgroup more compact.

The position of the aperture stop determines the telecentricity, andthus is important in terms of the optical arrangement. In order torealize a long back focus, telecentricity and the arrangement of anaperture stop that is positioned on the side having the longer conjugatedistance, it is important to provide as much positive power in thevicinity of the aperture stop as possible. In the present embodiment, inorder to satisfy the above-noted conditions, the positive power of thesecond lens group and the third lens group is larger than the positivepower of the fourth lens group 14.

A basic zoom lens generates minus distortion at the wide angle end, andplus distortion at the telephoto end that is larger than that of thewide angle end. In order to suppress distortion fluctuations due tochanges in magnification, it is desirable that the lens groups movemonotonically with changes in magnification. For example, in afour-group zoom that has positive, negative, positive and positiverefractive power, if the positive first lens group is fixed duringchanges in magnification, and if the magnification ratio of the negativesecond lens group moves about the position of equal magnification ratio,then the third lens group, which is positive, has the same position atthe wide angle end and at the telephoto end, and thus the maximum amountof movement is in between the wide angle end and the telephoto end. Inthis case, the distortion generated at the positive third lens group issubstantially the same at the wide angle end and the telephoto end, andthus it is not possible to suppress distortion fluctuation caused by achange in the magnification of the overall lens system.

In the present embodiment, the zoom lens is arranged so that themagnification ratio from the second lens group 112 to the fourth lensgroup 114 does not move about the position of equal magnification, andis arranged so as to be used at a minus reduction ratio. By arrangingthe zoom lens in this manner, in the present embodiment, when changingmagnification from the wide angle end to the telephoto end, the firstlens group 111 moves monotonically from the end having the longerconjugate distance to the end having the shorter conjugate distance, andthe lens groups from the second lens group 112 to the fourth lens group114 move monotonically from the end having the shorter conjugatedistance to the end having the longer conjugate distance. With theconfiguration noted above, it is possible to effectively suppressfluctuations in distortion as the magnification changes from the wideangle end to the telephoto end.

For zoom lenses in which the back focus fluctuates, if the aperture stopis not alterable with changes in magnification, then the F number willfluctuate. The amount by which the F number fluctuates is proportionalto the amount that the back focus fluctuates. The amount of F numberfluctuation may be reduced by reducing the amount of fluctuation in theback focus.

In the present embodiment, the absolute value of the magnification ratioof the lens groups from the second lens group 112 to the fourth lensgroup 114 is reduced. However, when this magnification ratio is reduced,the overall length of the lens increases, and it may be impossible toensure a long back focus. Accordingly, in the present embodiment, a longback focus may be ensured by configuring the second lens group 112 witha lens 112 a, which has negative refractive power, and a lens 112 b,which has positive refractive power, in the order as seen from the sidehaving the longer conjugate distance.

In the present embodiment, it is preferable to satisfy Expression (13)below:2.5<bfw/fw<4  (13)

where bfw is the air equivalent back focus when the wide angle end is atinfinity and where fw is the focal length of the zoom lens 110 at thewide angle end.

Expression (13) is an expression that prescribes the back focus at thewide angle end with respect to the focal length at the wide angle end,and also prescribes the necessary back focus of a projecting lens foruse in a projector Particularly, if reflecting type elements are used asthe spatial modulating elements, then in addition to a prism forcombining colors, a prism block for guiding the illuminating light isdisposed between the projecting lens and the spatial modulatingelements. Thus, the projecting lens for the projector requires a longback focus. When the lower limit of Expression (13) is not met, it isnot possible to obtain the necessary space between the projecting lensand the spatial modulating elements, and it may not be possible toconfigure the projector. When the upper limit is exceeded, the overalllength and outside diameter of the lens increases, and it is notpossible to make the lens more compact.

A preferable configuration of the present embodiment is described belowin terms of optical performance. First, the first lens of the secondlens group 112, as seen from the side having the longer conjugatedistance, has negative refractive power, the second lens, as seen fromthe side having the longer conjugate distance, has positive refractivepower, and the second lens group 112 is constituted by at least threelenses. Thus, it is possible to ensure a long back focus by configuringthe second lens group 112 with lenses having negative and positiverefractive power, as seen from the side having the longer conjugatedistance. It should be noted that in the example in FIG. 1, the secondlens group 112 is constituted by three lenses, however provided that anegative lens and a positive lens are disposed in that order from theside having the longer conjugate distance, the second lens group 112 maybe constituted by four or more lenses.

Next, when there is a change in magnification from the wide angle end tothe telephoto end, the second lens group 112 and the fourth lens group114 move in the same way on the optical axis, from the side having theshorter conjugate distance to the side having the longer conjugatedistance. Since the aperture stop 17 is disposed within the second lensgroup 112, because the fourth lens group 114 moves in the same way asthe second lens group 112, there is no change in telecentricity with thechange in magnification form the wide angle end to the telephoto end. Itis also possible to simplify the construction of the lens barrel, andthis construction has the advantage of reducing costs while maintainingprecision.

Next, it is preferable to satisfy Expressions (14) to (17) below:−0.45<fw/f1 g<−0.3  (14)0.01<fw/f2 g<0.3  (15)0.18<fw/f3 g<0.29  (16)0.05<fw/f4 g<0.2  (17)

where f1g is the focal length of the first lens group 111, where f2g isthe focal length of the second lens group 112, where f3g is the focallength of the third lens group 113, where f4g is the focal length of thefourth lens group 114 and where fw is the focal length of theabove-noted zoom lens at the wide angle end,

Expression (14) is an expression that prescribes the focal length of thefirst lens group 111 by the ratio to the focal length at the wide angleend, and thus, when the lower limit is not met, the Petzval Sum may notbe able to be corrected, and curvature of the field and astigmaticaberration increases. When the upper limit is exceeded, it may beimpossible to ensure the back focus. If an attempt is made to ensure theback focus, then the optical length of the entire zoom lens and theoutside diameter of the first lens group increases.

Expression (15) is an expression that prescribes the focal length of thesecond lens group 112 by the ratio to the focal length of the zoom lensat the wide angle end. When the lower limit is not met, coma aberrationincreases, and when the upper limit is exceeded, it becomes impossibleto ensure the back focus.

Expression (16) is an expression that prescribes the focal length of thethird lens group 113 by the ratio to the focal length of the zoom lensat the wide angle end. When the lower limit is not met, the position ofthe aperture stop moves toward the side having the shorter conjugatedistance, and the outer diameter of the first lens group 111 increases.When the upper limit is exceeded, it may come be impossible to correctspherical aberration.

Expression (17) is an expression that prescribes the focal length of thefourth lens group by the ratio to the focal length of the zoom lens atthe wide angle end. When the lower limit is not met, it may beimpossible to ensure the back focus. When the upper limit is exceeded,it may be impossible to correct distortion and lateral chromaticaberration.

Next, it is preferable to satisfy Expressions (18) and (19) below:−0.018<(1/f1 /abe1 )/(1/frear)<0.0  (18)1.70<nd11<1.79  (19)

where f1 is the focal length of the front negative lens 111 a when seenfrom the end having the longer conjugate distance, where abe1 is theAbbe number, where nd11 is the refractive index at the d line, and wherefrear is the synthetic focal length from the second lens group 112 tothe fourth lens group 114,

When the lens groups from the second lens group 112 to the fourth lensgroup 114 correct chromatic aberration, blue lateral chromaticaberration is over-corrected. The lens that cancels out thisover-correction of blue lateral chromatic aberration is the negativelens 111 a that is at the front as seen from the side having the longerconjugate distance.

Expression (18) represents the relationship between the amount of bluelateral chromatic aberration that is generated in the front negativelens, as seen from the side having the longer conjugate distance, andthe amount of over-correction of blue lateral chromatic aberration bythe lens groups from the second lens group to the fourth lens group.When the lower limit is not met, correction of the blue lateralchromatic aberration and the red lateral chromatic aberration isinsufficient, and when the upper limit is exceeded, the blue lateralchromatic aberration increases due to over-correction.

It is preferable that the refractive index of the front negative lens111 a, as seen from the side having the longer conjugate distance ishigh, and that its Abbe number is small. However, glass material such asis noted above is characterized by degradation of its internaltransmittance. Expression (19) prescribes the refractive index of thefront negative lens 111 a. When the lower limit is not met, it becomesimpossible to make the over-correction of the blue lateral chromaticaberration smaller, and when the upper limit is exceeded, the internaltransmittance decreases and the color balance worsens.

Next, the configuration of the four lenses as seen from the side havingthe shorter conjugate distance (114 a to 114 d) is, in the order fromthe side having the longer conjugate distance, a negative meniscus lens114 a whose convex surface faces the side having the longer conjugatedistance, a positive lens 114 b, a negative meniscus lens 114 c whoseconvex surface faces the side having the shorter conjugate distance anda positive lens 114 d. It is preferable to satisfy Expressions (20) to(22) below:nd4>1.75  (20)vd4>35  (21)1<f4 r/bfw<4  (22)

where nd4 is the refractive index of the negative meniscus lens 114 onthe side having the longer conjugate distance at the d line, where vd4is the Abbe number, where f4r is the focal length of the four lenses onthe side having the shorter conjugate distance and where bfw is the airequivalent back focus at the wide angle end that does not include prismsor the cover glass.

By facing the convex surfaces of the two negative meniscus lenses indifferent directions, they may be advantageously applied to reducelateral chromatic aberration and distortion aberration. The negativemeniscus lens 114 a, whose convex surface faces the side having thelonger conjugate distance, may be used effectively to correct distortionaberration, and the negative meniscus lens 114 c, whose convex surfacefaces the side having the shorter conjugate distance may be usedeffectively to correct lateral chromatic aberration.

Expression (20) represents the refractive index, at the d line, of thenegative meniscus lens that is on the side that has the longer conjugatedistance, and when the lower limit is not met, the curvature of thefield increases. Expression (9) represents the Abbe number of thenegative meniscus lens on the side that has the longer conjugatedistance, and when the lower limit is not met, the lateral chromaticaberration increases. Furthermore, in Expression (21), it is even morepreferable to satisfy vd4>40.

Expression (22) represents the situation in which the focal length ofthe four lenses, which are on the side having the shorter conjugatedistance, are larger than the air equivalent back focus that does notinclude the prism or the cover glass when at the wide angle end, and itshows use of the lens in a manner in which when an F number light beamis incident on the four lenses from the side having the shorterconjugate distance the light converges toward the side having theshorter conjugate distance. When the lower limit is not met, the outerdiameter of the lens on the side having the longer conjugate distanceincreases, and the distortion aberration and the lateral chromaticaberration increase. When the upper limit is exceeded, the overalllength of the lens increases, and it may be impossible to ensure theback focus.

Next, it is preferable that all the lenses having positive refractivepower that constitute the third lens group 113 and the fourth lens group114 are configured with an Abbe number of at least 80. The third lensgroup 113 and the fourth lens group 114 have positive refractive power,the principal light beam is significantly bent in order to ensuretelecentricity, and there is significant chromatic aberration. Lateralchromatic aberration is particularly large. If all the lenses havingpositive refractive power that constitute the third lens group 113 andthe fourth lens group 114 have an Abbe number of at least 80, then it ispossible to reduce lateral chromatic aberration to a small value.

It should be noted that the configuration that satisfies the above-notedExpressions (18) and (19), and the configuration that satisfies theabove-noted Expressions (20) to (22) have been described under theprecondition that each may be applied to the configuration thatsatisfies Expression (13), however the effect of satisfying theseexpressions, such as was described above, may be obtained even with aconfiguration that does not satisfy the above-noted Expression (13).

WORKING EXAMPLE 4

Working Example 4 according to Embodiment 4 is described below. The lensconfiguration of Working Example 4 is the same as the configuration inFIG. 17 and FIG. 18, and is a design example in which F_(NO)=2.5 at thewide angle end, focal length f=27.84 and a half angle of view is 30.9°.The values of the above noted Expressions (13) to (22) in WorkingExample 4 are as follows:bfw/fw=2.78  Expression (13)fw/f1 g=−0.39  Expression (14)fw/f2 g=0.277  Expression (15)fw/f3 g=0.228  Expression (16)fw/f4 g=0.09  Expression (17)(1/f1 /abe1 )/(1/frear)=−0.011  Expression (18)nd11=1.784  Expression (19)nd4=1.834  Expression (20)vd4=37.3  Expression (21)f4 r/bfw=3.94.  Expression (22)

Next, specific figures are shown in Table 9, and zoom data is shown inTable 10. In Table 9, ri (mm) is the radius of curvature of the lenssurface, di (mm) is the lens thickness or the interval between lenses,ni is the refractive index of the lens at the d line, and vi is the Abbenumber of the lens at the d line. This is the same in Tables 11 and 13below. In the example of Table 9, r1 to r12 is the first lens group, r13to r19 is the second lens group, r20 to r27 is the third lens group, r28to r35 is the fourth lens group, and r15 is the aperture stop.

TABLE 9 ri di Nd νd r1 = 95.000 d1 = 3.3 n1 = 1.78472 v1 = 25.72 r2 =59.510 d2 = 8.4 r3 = 102.250 d3 = 13.2 n2 = 1.58913 v2 = 61.25 r4 =−487.000 d4 = 0.8 r5 = 66.870 d5 = 2.5 n3 = 1.49700 v3 = 81.61 r6 =33.580 d6 = 15.0 r7 = −221.560 d7 = 2.0 n4 = 1.49700 v4 = 81.61 r8 =45.200 d8 = 7.0 r9 = 260.000 d9 = 2.0 n5 = 1.49700 v5 = 81.61 r10 =65.240 d10 = 0.9 r11 = 46.300 d11 = 7.0 n6 = 1.71736 v6 = 29.50 r12 =123.000 d12 = Variabie r13 = 67.700 d13 = 2.6 n7 = 1.88300 v7 = 40.80r14 = 35.650 d14 = 22.8 r15 = 0.000 d15 = 1.0 r16 = 131.500 d16 = 4.3 n8= 1.71736 v8 = 29.50 r17 = −90.800 d17 = 15.2 r18 = −54.200 d18 = 9.8 n9= 1.56883 v9 = 56.04 r19 = −41.540 d19 = Variable r20 = −48.750 d20 =2.6 n10 = 1.83500 v10 = 42.98 r21 = 133.500 d21 = 0.5 r22 = 169.000 d22= 8.6 n11 = 1.49700 v11 = 81.61 r23 = −62.140 d23 = 0.5 r24 = −6982.000d24 = 5.7 n12 = 1.49700 v12 = 81.61 r25 = −81.800 d25 = 0.3 r26 = 70.600d26 = 10.9 n13 = 1.45650 v13 = 90.27 r27 = −120.000 d27 = Variable r28 =380.000 d28 = 2.7 n14 = 1.83400 v14 = 37.34 r29 = 60.100 d29 = 4.5 r30 =257.000 d30 = 7.5 n15 = 1.49700 v15 = 81.61 r31 = −102.000 d31 = 4.7 r32= −46.300 d32 = 2.5 n16 = 1.83400 v16 = 37.34 r33 = −50.900 d33 = 0.7r34 = 168.290 d34 = 10.0 n17 = 1.45650 v17 = 90.27 r35 = −93.800 d35 =Variable r36 = 0.000 d36 = 88.0 n18 = 1.51680 v18 = 64.17

TABLE 10 Wide angle end Telephoto end d12 46.8 5.9 d19 10.6 9.18 d273.17 4.61 d35 14.41 21.98

The charts in FIG. 19 show the spherical aberration (mm), astigmaticaberration (mm) and distortion aberration (%) at the wide angle end ofWorking Example 4, and these are the same as in FIG. 7 and FIG. 11below. The charts in FIG. 20 show the spherical aberration (mm),astigmatic aberration (mm) and distortion aberration (%) at thetelephoto end of Working Example 4, and these are the same as in FIG. 24and FIG. 28 below. As can be seen in FIG. 19 and FIG. 20, the zoom lensaccording to Working Example 4 shows favorable performance with respectto aberration.

WORKING EXAMPLE 5

FIG. 21 is a view of a configuration of the wide angle end of a zoomlens according to Working Example 5. FIG. 22 is a view of aconfiguration of the telephoto end of the zoom lens shown in FIG. 21. Azoom lens 200 shown in FIG. 21 has a four-group configuration, and isprovided with a first lens group 201 that has negative refractive power(lens 201 a to 201 f), a second lens group 202 that has positiverefractive power (lens 202 a to 202 c), a third lens group 203 that haspositive refractive power (lens 203 a to 203 c) and a fourth lens group204 that has positive refractive power (lens 204 a to 204 d), in theorder as seen from the side with the longer conjugate distance.

Furthermore, when changing magnification from the wide angle end (FIG.21) to the telephoto end (FIG. 22), the first lens group 201 moves tothe side having the shorter conjugate distance, the second lens group202 moves to the side having the longer conjugate distance, the thirdlens group 203 moves to the side having the longer conjugate distance,and the fourth lens group 204 moves to the side having the longerconjugate distance.

Working Example 5 is a design example having F_(NO)=2.3 at the wideangle end, a focal length f=21.32 and a half angle of view of 30.9°. Thevalues of the above noted Expressions (13) to (22) in Working Example 5are as follows:bfw/fw=3.66  Expression (13)fw/f1 g=−0.37  Expression (14)fw/f2 g=0.01  Expression (15)fw/f3 g=0.222  Expression (16)fw/f4 g=0.175  Expression (17)(1/f1 /abe1 )/(1/frear)=−0.009  Expression (18)nd11=1.784  Expression (19)nd4=1.834  Expression (20)vd4=37.3  Expression (21)f4r/bfw=1.56.  Expression (22)

Next, specific figures are shown in Table 11, and zoom data is shown inTable 12. In the example of Table 11, r1 to r12 is the first lens group,r13 to r19 is the second lens group, r20 to r25 is the third lens group,r26 to r33 is the fourth lens group, and r17 is the aperture stop.

TABLE 11 ri di Nd νd r1 = 92.300 d1 = 3.2 n1 = 1.78472 v1 = 25.72 r2 =61.092 d2 = 7.6 r3 = 100.539 d3 = 13.0 n2 = 1.58913 v2 = 61.25 r4 =−639.005 d4 = 0.5 r5 = 82.393 d5 = 2.7 n3 = 1.49700 v3 = 81.61 r6 =35.903 d6 = 10.4 r7 = 127.653 d7 = 2.3 n4 = 1.49700 v4 = 81.61 r8 =42.052 d8 = 24.0 r9 = −89.136 d9 = 2.3 n5 = 1.49700 v5 = 81.61 r10 =79.123 d10 = 0.7 r11 = 57.850 d11 = 10.0 n6 = 1.71736 v6 = 29.50 r12 =199.948 d12 = Variable r13 = 144.430 d13 = 2.2 n7 = 1.88300 v7 = 40.80r14 = 46.361 d14 = 19.0 r15 = 109.950 d15 = 5.2 n8 = 1.71736 v8 = 29.50r16 = −89.666 d16 = 15.1 r17 = 0.000 d17 = 10.0 r18 = −67.922 d18 = 7.5n9 = 1.56883 v9 = 56.04 r19 = −143.285 d19 = Variable r20 = 114.850 d20= 2.1 n10 = 1.83500 v10 = 42.98 r21 = 48.013 d21 = 4.2 r22 = 61.255 d22= 11.0 n11 = 1.49700 v11 = 81.61 r23 = −196.811 d23 = 0.4 r24 = 110.430d24 = 8.3 n12 = 1.45650 v12 = 90.27 r25 = −91.881 d25 = Variable r26 =369.544 d26 = 2.1 n13 = 1.83400 v13 = 37.34 r27 = 53.799 d27 = 3.5 r28 =452.303 d28 = 5.9 n14 = 1.49700 v14 = 81.61 r29 = −69.395 d29 = 3.6 r30= −39.470 d30 = 1.8 n15 = 1.83400 v15 = 37.34 r31 = −47.130 d31 = 0.5r32 = 92.644 d32 = 11.5 n16 = 1.45650 v16 = 90.27 r33 = −51.636 d33 =Variable r34 = 0.000 d34 = 93.5 n17 = 1.51680 v17 = 64.17

TABLE 12 Wide angle end Telephoto end d12 40.8 4.5 d19 8.15 6.62 d252.43 3.95 d33 10.88 16.03

The charts in FIG. 23 and FIG. 24 respectively show the aberrationcharts of the wide angle end of Working Example 5 and the telephoto end.It can be seen that the zoom lens according to Working Example 5 showsfavorable performance with respect to aberration.

WORKING EXAMPLE 6

FIG. 25 is a view of a configuration of the wide angle end of a zoomlens according to Working Example 6. FIG. 26 is a view of aconfiguration of the telephoto end of the zoom lens shown in FIG. 25. Azoom lens 300 shown in FIG. 25 has a four-group configuration, and isprovided with a first lens group 301 that has negative refractive power(lenses 301 a to 301 f), a second lens group 302 that has positiverefractive power (lenses 302 a to 302 c), a third lens group 303 thathas positive refractive power (lenses 303 a to 303 c) and a fourth lensgroup 304 that has positive refractive power (lenses 304 a to 304 d), inthe order as seen from the side with the longer conjugate distance.

Furthermore, when changing magnification from the wide angle end (FIG.25) to the telephoto end (FIG. 26), the first lens group 301 moves tothe side having the shorter conjugate distance, the second lens group302 moves to the side having the longer conjugate distance, the thirdlens group 303 moves to the side having the longer conjugate distance,and the fourth lens group 204 moves to the side having the longerconjugate distance.

Working Example 6 is a design example having F_(NO)=2.4 at the wideangle end, a focal length f=28.93 and a half angle of view of 30.9°. Thevalues of the above noted Expressions (13) to (22) in Working Example 6are as follows:bfw/fw=2.79  Expression (13)fw/f1 g=−0.38  Expression (14)fw/f2 g=0.29  Expression (15)fw/f3 g=0.245  Expression (16)fw/f4 g=0.096  Expression (17)(1/f1 /abe1 )/(1/frear)=−0.012  Expression (18)nd11=1.784  Expression (19)nd4=1.834  Expression (20)vd4=37.3  Expression (21)f4 r/bfw=3.75.  Expression (22)

Next, specific figures are shown in Table 13, and zoom data is shown inTable 14. In the example of Table 13, r1 to r12 is the first lens group,r13 to r19 is the second lens group, r20 to r27 is the third lens group,r28 to r35 is the fourth lens group, and r15 is the aperture stop.

TABLE 13 ri di Nd νd r1 = 100.250 d1 = 3.4 n1 = 1.78472 v1 = 25.72 r2 =62.092 d2 = 8.7 r3 = 102.718 d3 = 13.7 n2 = 1.58913 v2 = 61.25 r4 =−412.312 d4 = 0.8 r5 = 90.301 d5 = 2.6 n3 = 1.49700 v3 = 81.61 r6 =37.482 d6 = 15.6 r7 = −153.337 d7 = 2.1 n4 = 1.49700 v4 = 81.61 r8 =58.722 d8 = 7.3 r9 = 337.645 d9 = 2.1 n5 = 1.49700 v5 = 81.61 r10 =76.109 d10 = 0.9 r11 = 57.062 d11 = 7.3 n6 = 1.71736 v6 = 29.50 r12 =187.320 d12 = Variable r13 = 58.649 d13 = 2.7 n7 = 1.88300 v7 = 40.80r14 = 35.295 d14 = 23.7 r15 = 0.000 d15 = 1.0 r16 = 216.102 d16 = 4.5 n8= 1.71736 v8 = 29.50 r17 = −84.450 d17 = 15.8 r18 = −53.730 d18 = 10.2n9 = 1.56883 v9 = 56.04 r19 = −39.951 d19 = Variable r20 = −44.802 d20 =2.7 n10 = 1.83500 v10 = 42.98 r21 = −477.367 d21 = 4.1 r22 = 144.976 d22= 9.0 n11 = 1.49700 v11 = 81.61 r23 = −55.671 d23 = 0.5 r24 = 789.245d24 = 5.9 n12 = 1.49700 v12 = 81.61 r25 = −90.970 d25 = 0.3 r26 = 72.267d26 = 11.3 n13 = 1.45650 v13 = 90.27 r27 = −217.365 d27 = Variable r28 =752.473 d28 = 2.8 n14 = 1.83400 v14 = 37.34 r29 = 60.655 d29 = 4.7 r30 =287.456 d30 = 7.8 n15 = 1.49700 v15 = 81.61 r31 = −85.523 d31 = 4.9 r32= −45.608 d32 = 2.6 n16 = 1.83400 v16 = 37.34 r33 = −55.469 d33 = 0.7r34 = 166.870 d34 = 10.4 n17 = 1.45650 v17 = 90.27 r35 = −72.557 d35 =Variable r36 = 0.000 d36 = 91.5 n18 = 1.51680 v18 = 64.17

TABLE 14 Wide angle end Telephoto end d12 48.7 6.1 d19 11.0 9.4 d27 3.34.95 d35 15.5 23.4

The charts in FIG. 27 and FIG. 28 respectively show the aberrationcharts of the wide angle end of Working Example 6 and the telephoto end.It can be seen that the zoom lens according to Working Example 6 showsfavorable performance with respect to aberration.

EMBODIMENT 5

FIG. 29 shows a view of a configuration of a wide angle lens accordingto Embodiment 5. A wide angle lens 400 shown in the diagram has athree-group configuration that is provided, as seen from the side havingthe longer conjugate distance, with a first lens group 401 that hasnegative refractive power (lens 401 a to 401 f), a second lens group 402(lens 402 a to 402 c) and a third lens group 403 that has positiverefractive power (lens 403 a to 403 g). The refractive power of thesecond lens group 402 is weaker than that of the first lens group 401and the third lens group 403. More specifically, it is preferable thatthe refractive power of the second lens group 402 is less than about ⅕the refractive power of the first lens group 401 and the third lensgroup 403, and may be one tenth of the power or less. This is the samein the configuration shown in FIG. 31 and FIG. 33 below.

Numeral 14 denotes a glass block such as a prism. Numeral 15 denotes animage surface, and in the case of an image-taking system denotes film orCCDs, and in the case of a projection device, it denotes LCDs, forexample, which are spatial modulating elements. Furthermore, an aperturestop 16 is disposed between the second lens group 402 and the third lensgroup 403. It should be noted that in the example of this diagram, theside with the longer conjugate distance is the side that is opposite theimage surface 15 (similarly with FIG. 31 and FIG. 33).

When the wide angle lens 400 changes magnification from near to far, thefirst lens group 401 and the third lens group 403 move toward the sidehaving the shorter conjugate distance, and the second lens group 402moves so as to reduce an interval d12 between the second lens group 402and the first lens group 401.

The first lens group 401 is constituted by six lenses, a negative lens401 a, a negative lens 401 b, a positive lens 401 c, a negative lens 401d, a negative lens 401 e and a positive lens 401 f, in that order fromthe side having the longer conjugate distance. The second lens group 402is constituted by three lenses, a negative lens 402 a, a positive lens402 b and a positive lens 402 c, in that order from the side having thelonger conjugate distance. When the projection distance changes, thesecond lens group 402 changes the interval between it and the first lensgroup 401 and the third lens group 403 to correct aberrations.

The third lens group 403 has positive refractive power. Since the thirdlens group 403 has a significant influence on distortion and lateralchromatic aberration, it is configured to suppress these aberrationseffectively.

Thus, a wide angle lens that has a long back focus is realized byconfiguring the wide angle lens 400 with three groups, the first lensgroup 401, which has negative refractive power, the second lens group402, which has weak refractive power, and the third lens group 403,which has positive refractive power, in that order as seen from the sidehaving the longer conjugate distance.

The wide angle lens 400 is described more specifically below. The wideangle lens 400 is based on a reverse telephoto-type (retro focus-type)lens that is constituted by lens groups having negative, positiverefractive power. Reverse telephoto-type lenses are characterized inthat a long back focus may be obtained easily, however its opticalperformance is easily altered with changes in the projection distance.Thus, in the present embodiment, design alterations have been made tothe focusing method with respect to changes in projection distance.

More specifically, when the magnification is changed from near to far,the wide angle lens 400 of the present embodiment is set such that inaddition to moving all the lens groups from the first lens group 401 tothe third lens group 403 in the direction of the optical axis, thesecond lens group 402 is provided with movement that is different fromthe movement of the first lens group 401 and the third lens group 403.That is to say, the second lens group 402 is moved so as to reduce theair space between the first lens group 401 and the second lens group402, and to increase the air space between the second lens group 402 andthe third lens group 403. Thus, as is described in detail below, thewide angle lens 400 is arranged such that its optical performance doesnot change with respect to changes in the projection distance.

The second lens group 402 has refractive power that is weaker than therefractive power of the first lens group 401 and the third lens group403. The second lens group 402 is constituted by the lens 402 a, whichis a concave lens, and the lenses 402 b and 402 c, which are convexlenses, however a glass material that has a low refractive power is usedfor the concave lens, and a glass material that has a high refractiveindex is used for the convex lenses. Since the refractive power of thesecond lens group 402 is weak, although the power of the convex lensesand the concave lens is the same, the concave lens that has a lowrefractive index generates a large aberration, and as a result, thesecond lens group 402 generates positive spherical aberration.

The first lens group 401 has negative refractive power, but since theaxial ray is low and there are many lenses provided to constitute thefirst lens group 401, the first lens group corrects the sphericalaberration to a sufficiently small value. Because the axial ray from thethird lens group 403 passes through a high position, and the Petzval Summay be suppressed to a small value, a glass material that has a lowrefractive index is used for the convex lens and thus the third lensgroup 403 generates negative spherical aberration. In this way, thespherical aberration of the first lens group 401 is ±zero, that of thesecond lens group 402 is positive, and that of the third lens group 403is negative, and thus the entire body from the first lens group 401 tothe third lens group 403 balances the spherical aberration.

In order to suppress the overall aberration of the entire wide anglelens 400 to a small value, the aberration that is generated by the lensgroups and the interval between the lens groups is important. Whenconsidering the side having the longer conjugate distance, the firstlens group 11 suppresses the spherical aberration to a small value, andthus even if the interval with the lens group 402 changes, there is nochange in the spherical aberration of the entire wide angle lens 400.

On the other hand, because the second lens group 402 has positivespherical aberration, if the interval with the third lens group 403increases, then the influence of the plus spherical aberration of thesecond lens group 402 increases and the entire wide angle lens 400attains plus spherical aberration.

However, the height of the principal ray of the second lens group 402 ishigh, and the second lens group 402 has a greater influence onastigmatic aberration than on spherical aberration, and thusfluctuations in aberration with movement of the second lens group 402appear in prominent astigmatic aberration rather than sphericalaberration. Thus, if such fluctuations in astigmatic aberration due tomovement of the second lens group 402 are utilized, then by moving thesecond lens group 402, it is possible to correct fluctuations inaberration caused by fluctuations in the projection distance. If, forexample, the second lens group 402 is fixed with respect to the firstlens group 401 and the third lens group 403, then astigmatic aberrationis generated when the magnification is changed from near to far, butthis cannot be corrected.

That is to say, when the magnification is changed from near to far, whenthe lens groups from the first lens group 401 to the third lens group403 are moved to the side having the shorter conjugate distance, thewide angle lens 400 of the present embodiment is able to handlefluctuations in the projection distance by moving the second lens group402 along the optical axis such that the air space with the first lensgroup 401 is reduced and can correct astigmatic aberration.

In addition to this configuration, if the air equivalent back focus atinfinity of the wide angle lens 400 is bf, and the focal length of thewide angle lens is f, then by satisfying Expression (23) below, it ispossible to achieve a long back focus while realizing a wide angle lensin which changes in performance due to changes in projection distanceare decreased4<bf/f<6.  (23)

Expression (23) prescribes the ratio of back focus with respect to thefocal length of the entire lens system, and prescribes the back focusrequired for the projecting lens used in the projector.

In particular, if the projector uses reflective-type elements for thespatial modulating elements, then in addition to the color combiningprism, a prism block for guiding the illuminated light is disposedbetween the projecting lens and the spatial modulating elements. Thus, along back focus is required for the projecting lens used in a projector.

More specifically, when the lower limit of Expression (23) is not met,it may be impossible to obtain the necessary space between theprojecting lens and the spatial modulating elements, and it may beimpossible to configure the projector. Furthermore, if the upper limitis exceeded, then the overall length, and the outer diameter increases,and it may be impossible to make the lens more compact.

The present embodiment is configured such that when changingmagnification from near to far, all the lens groups from the first lensgroup 401 to the third lens group 403 move in the direction of theoptical axis, and such that the second lens group 402 moves differentlyfrom the first lens group 401 and the third lens group 403. In thisconfiguration, when changing the magnification from near to far, it ispreferable that the first lens group 401 and the third lens group 403move in the same way on the optical axis. Accordingly, the first lensgroup 401 and third lens group 403 move together during focusing whilebeing fixed with respect to each other, and thus it is possible tosimplify the lens barrel construction.

A configuration of the present embodiment that is preferable in terms ofoptical performance is described below. When the focal length of thefirst lens group 401 is f1g, the focal length of the second lens group402 is f2g, the focal length of the third lens group 403 is f3g, and thefocal length of the wide angle lens 400 is f, it is preferable that thepresent embodiment satisfies Expressions (24) to (26) below−0.4<f/f1 g<−0.15  (24)−0.2<f/f2 g<0.05  (25)0.15<f/f3 g<0.25.  (26)

By satisfying Expressions (24) to (26), it is possible to realize a wideangle lens that is compact, and in which distortion aberration andchromatic aberration is corrected favorably. Expression (24) is anexpression that prescribes the focal length of the first lens group 401by the ratio to the focal length of the entire lens, and when the lowerlimit is not met, it may be impossible to correct the Petzval Sum, andcurvature of field and astigmatic aberration increase. When the upperlimit is exceeded, it may be impossible to ensure the back focus, and ifan attempt is made to ensure the back focus, then the entire opticallength of the zoom lens increases, as does the outer diameter of thefirst lens group 401.

Expression (25) is an expression that prescribes the focal length of thesecond lens group 402 by the ratio to the focal length of the entirelens. When the lower limit is not met, it may become impossible tocorrect astigmatic aberration that occurs due to changes in theprojection distance by moving the second lens group 402. When the upperlimit is exceeded, it may be impossible to ensure the back focus.

Expression (26) is an expression that prescribes the focal length ofthird lens group 403 the ratio to the focal length of the entire lens.When the lower limit is not met, the entire length of the lensincreases, and when the upper limit is exceeded, it may be impossible toensure the back focus, and it may also be impossible to correctdistortion aberration or lateral chromatic aberration.

Next, it is preferable that the wide angle lens 400 of the presentembodiment satisfies Expressions (27) and (28) below:−0.025<(1/f1 /abe1 )/(1/f3 g)<−0.008  (27)1.7<nd11<1.79  (28)

where f1 is the focal length of the front negative lens, as seen fromthe side having the longer conjugate distance (the lens 401 a of thefirst lens group 401), where abe1 is the Abbe number and where nd11 isthe refractive index at the d line, and where f3g is the focal length ofthe third lens group 403.

When the third lens group 403 corrects chromatic aberration, bluelateral chromatic aberration is over-corrected. The lens that cancelsout this over-correction of blue lateral chromatic aberration is thenegative lens 401 a, which is at the front as seen from the side havingthe longer conjugate distance. The amount of blue lateral chromaticaberration that is generated by the negative lens 401 a cancels out theover-correction of the blue lateral chromatic aberration that isgenerated at the third lens group and it is possible to suppress thelateral chromatic aberration to a small value.

Expression (27) represents the relationship between the amount of bluelateral chromatic aberration that is generated by the negative lens 401(f1/abe1) to the amount of over-correction of the blue lateral chromaticaberration that is generated by the third lens group 403 (f3g). When thelower limit is not met, correction of the blue lateral chromaticaberration and correction of the red lateral chromatic aberration becomeinsufficient. When the upper limit is exceeded, over-correction of theblue lateral chromatic aberration increases.

It is preferable that the refractive index of the negative lens 401 a ishigh, and that it has a small Abbe number. However, a glass materialsuch as was noted above is characterized in that the internaltransmittance degrades. Expression (28) prescribes the refractive indexof the negative lens 401 a. When the lower limit is not met, it may beimpossible to reduce the over-correction of the blue lateral chromaticaberration, and when the upper limit is exceeded, the interaltransparency ratio decreases, and the color balance worsens.

Next, the four lenses on the side having the shorter conjugate distance(the lenses 403 d to 403 g of the third lens group) are arranged inorder, from the side having the longer conjugate distance, of a negativemeniscus lens whose convex surface faces the side having the longerconjugate distance (the lens 403 d), a positive lens (the lens 403 e), anegative meniscus lens whose convex surface faces the side having theshorter conjugate distance (the lens 403 f) and a positive lens (thelens 403 g). It is preferable to satisfy the Expressions (29) to (31)below:nd4>1.75  (29)vd4>35  (30)1<f4 r/bf<1.5  (31)

where nd4 is the refractive index at the d line of the negative meniscuslens 403 d, which is on the side having the longer conjugate distance,where vdf is the Abbe number, where f4r is the focal length of the fourlenses from the side having the shorter conjugate distance (lenses 403 dto 403 g), and where bf is the air equivalent back focus that does notinclude the prisms or the cover glass.

With this configuration, it is possible to suppress distortionaberration and lateral chromatic aberration to a small value. The lenseson the side having the shorter conjugate distance generate largedistortion aberrations and lateral chromatic aberrations, and theirpower and shape are very important for correcting this.

f4r/bf represents the ratio of the focal length of the four lenses onthe side having the shorter conjugate distance to the air equivalentback focus that does not include prisms or a cover glass, and relates tothe correction of the distortion aberration and the lateral chromaticaberration, the entire lens length, and the outer diameter of the lenson the side having the longer conjugate distance. By facing the convexsurfaces of the two negative meniscus lenses in different directions,they may be applied advantageously to reduce lateral chromaticaberration and distortion aberration. That is to say, the negativemeniscus lens whose convex surface faces the side having the longerconjugate distance may be used effectively to correct distortionaberration and the negative meniscus lens whose convex surface faces theside having the shorter conjugate distance may be used effectively tocorrect lateral chromatic aberration.

Expression (29) and (30) are expressions that prescribe the conditionsfor suppressing over-correction of blue lateral chromatic aberrationthrough the refractive index and the Abbe number of the negativemeniscus lens on the side having the longer conjugate distance.Expression (29) represents the refractive index of the negative meniscuslens on the side having the longer conjugate distance, and when thelower limit is not met, curvature of the field increases. Expression(30) represents the Abbe number of the negative meniscus lens on theside having the longer conjugate distance, and when the lower limit isnot met, the lateral chromatic aberration increases. Furthermore, inExpression (30), it is more preferable if vd4>40 is satisfied.

Expression (31) represents the situation in which the focal lengths ofthe four lenses 403 d to 403 g, which are on the side having the smallerconjugate distance, are larger than the air equivalent back focus thatdoes not include the prism or the cover glass. It shows use of the lensin a manner in which when an F number light beam is incident on the fourlenses from the side having the shorter conjugate distance the lightconverges toward the side having the shorter conjugate distance. Whenthe lower limit is not met, the outer diameter of the lens on the sidehaving the longer conjugate distance increases, and the distortionaberration and the lateral chromatic aberration increase. When the upperlimit is exceeded, the overall length of the lens increases, and itbecomes impossible to ensure the back focus.

Furthermore, it is preferable that all the lenses having positiverefractive power that constitute the third lens group 403 are configuredwith a refractive index at the d line of 1.65 or less. With thisconfiguration it is possible to suppress the Petzval Sum, and suppresscurvature of the field and astigmatic aberration to small values.

It should be noted that the configuration that satisfies the above-notedExpressions (27) and (28), and the configuration that satisfies theabove-noted Expressions (29) to (31) have been described under theprecondition that each may be applied to the configuration thatsatisfies Expression (23), however the effect of satisfying theseexpressions, such as was described above, may be obtained even with aconfiguration that does not satisfy the above-noted Expression (23).

WORKING EXAMPLE 7

Working Example 7 according to the present embodiment is shown usingspecific values. The wide angle lens configuration according to WorkingExample 7 is the same as the configuration shown in FIG. 29, and inWorking Example 7, F_(NO)=2.5, focal length f=14.85 and a half angle ofview of 38.7° is one design example. The values of Expressions (23) to(31) are shown below:bf/f=5.13  Expression (23)f/f1 g=−0.25  Expression (24)f/f2 g=−0.014  Expression (25)f/f3 g=0.217  Expression (26)(1/f1 /abe1 )/(1/f3 g)=−0.012  Expression (27)nd11=1.7847  Expression (28)nd4=1.835  Expression (29)vd4=42.98  Expression (30)f4 r/bf=1.32.  Expression (31)

Specific figures are shown below in Table 15, and zoom data is shown inTable 16. In Table 15, ri (mm) is the radius of curvature of the lenssurface, di (mm) is the lens thickness or the interval between lenses,ni is the refractive index of the lens at the d line, and vi is the Abbenumber of the lens at the d line. This is the same in Tables 17 and 19below. In the example of Table 15, r1 to r12 is the first lens group,r13 to r18 is the second lens group, r20 to r33 is the third lens groupand r19 is the aperture stop.

TABLE 15 ri di Nd νd r1 = 116.005 d1 = 5.7 n1 = 1.78472 v1 = 25.72 r2 =68.385 d2 = 16.6 r3 = 190.024 d3 = 4.5 n2 = 1.48749 v2 = 70.44 r4 =64.342 d4 = 16.0 r5 = 670.298 d5 = 13.5 n3 = 1.58913 v3 = 61.25 r6 =−140.817 d6 = 1.0 r7 = 360.131 d7 = 3.6 n4 = 1.49700 v4 = 81.61 r8 =60.146 d8 = 13.0 r9 = −294.790 d9 = 3.6 n5 = 1.49700 v5 = 81.61 r10 =79.331 d10 = 1.0 r11 = 52.996 d11 = 11.3 n6 = 1.74950 v6 = 35.04 r12 =78.491 d12 = Variable r13 = −225.898 d13 = 3.0 n7 = 1.61800 v7 = 63.39r14 = 39.064 d14 = 1.0 r15 = 40.507 d15 = 6.0 n8 = 1.71736 v8 = 29.50r16 = 57.648 d16 = 3.0 r17 = 954.029 d17 = 5.5 n9 = 1.58144 v9 = 40.89r18 = −57.015 d18 = Variable r19 = 0.000 d19 = 45.7 r20 = −403.167 d20 =6.3 n10 = 1.62004 v10 = 36.37 r21 = −61.612 d21 = 0.3 r22 = 222.298 d22= 7.6 n11 = 1.49700 v11 = 81.61 r23 = −64.222 d23 = 0.8 r24 = −52.091d24 = 2.6 n12 = 1.69680 v12 = 55.46 r25 = −388.361 d25 = 0.5 r26 =180.162 d26 = 3.3 n13 = 1.83500 v13 = 42.98 r27 = 45.123 d27 = 1.3 r28 =50.711 d28 = 12.0 n14 = 1.45650 v14 = 90.27 r29 = −47.813 d29 = 1.0 r30= −40.794 d30 = 3.2 n15 = 1.83400 v15 = 37.34 r31 = −76.870 d31 = 0.3r32 = 165.101 d32 = 10.0 n16 = 1.49700 v16 = 81.61 r33 = −48.307 d33 =Variable r34 = 0.000 d34 = 93.5 n17 = 1.51680 v17 = 64.17 r35 = 0.000d35 = 9.2

TABLE 16 Projection distance 1000 2000 3000 d0  1000 2000 3000 d12 79.7878.48 78.28 d18 64.04 65.34 65.54 d33 5.661 5.549 5.512

The charts in FIG. 30 show the spherical aberration (mm), astigmaticaberration (mm) and distortion aberration (%) of Working Example 7, andthese are the same for FIG. 32 and FIG. 34 below. As can be seen in FIG.30, the wide angle lens according to Working Example 7 shows favorableperformance with respect to aberration.

WORKING EXAMPLE 8

FIG. 31 is a view of a configuration of a wide angle lens according toWorking Example 8. A wide angle lens 500 shown in FIG. 31 has athree-group configuration, and is provided with a first lens group 501that has negative refractive power (lens 501 a to 501 f), a second lensgroup 502 that has weak refractive power (lens 502 a to 501 c) and athird lens group 503 that has positive refractive power (lens 503 a to503 g), in the order as seen from the side with the longer conjugatedistance. The basic configuration is the same as the wide angle lens 400shown in FIG. 29.

Working Example 8 has F_(NO)=2.5, a focal length f=15.33 and a halfangle of view of 36.7°. The values of the above noted Expressions (23)to (31) are shown below:bf/f=4.89  Expression (23)f/f1 g=−0.22  Expression (24)f/f2 g=−0.016  Expression (25)f/f3 g=0.192  Expression (26)(1/f1 /abe1 )/(1/f3 g)=−0.0128  Expression (27)nd11=1.7847  Expression (28)nd4=1.835  Expression (29)vd4=37.2  Expression (30)f4 r/bf=1.28.  Expression (31)

Specific figures are shown below in Table 17, and zoom data is shown inTable 18. In the example of Table 17, r1 to r12 is the first lens group,r13 to r18 is the second lens group, r20 to r33 is the third lens groupand r19 is the aperture stop.

TABLE 17 ri di Nd νd r1 = 103.779 d1 = 5.8 n1 = 1.78472 v1 = 25.72 r2 =65.775 d2 = 18.1 r3 = 158.849 d3 = 4.4 n2 = 1.49700 v2 = 81.61 r4 =75.613 d4 = 14.5 r5 = 405.447 d5 = 12.8 n3 = 1.58913 v3 = 61.25 r6 =−163.568 d6 = 1.0 r7 = 294.509 d7 = 3.5 n4 = 1.49700 v4 = 81.61 r8 =60.801 d8 = 13.8 r9 = −396.870 d9 = 3.5 n5 = 1.49700 v5 = 81.61 r10 =85.754 d10 = 1.0 r11 = 51.094 d11 = 12.2 n6 = 1.74950 v6 = 35.04 r12 =66.005 d12 = Variable r13 = 14684.672 d13 = 5.3 n7 = 1.61800 v7 = 63.40r14 = 38.696 d14 = 1.0 r15 = 51.369 d15 = 12.0 n8 = 1.71736 v8 = 29.50r16 = 109.984 d16 = 21.8 r17 = 226.742 d17 = 4.3 n9 = 1.49700 v9 = 81.60r18 = 126.175 d18 = Variable r19 = 0.000 d19 = 15.6 r20 = 80.270 d20 =6.6 n10 = 1.64769 v10 = 33.84 r21 = 1067.340 d21 = 18.5 r22 = −414.593d22 = 8.0 n11 = 1.49700 v11 = 81.61 r23 = −37.826 d23 = 1.0 r24 =−37.398 d24 = 3.5 n12 = 1.69680 v12 = 55.46 r25 = −77.336 d25 = 31.0 r26= 100.586 d26 = 5.1 n13 = 1.83400 v13 = 37.20 r27 = 65.686 d27 = 1.9 r28= 114.898 d28 = 11.0 n14 = 1.45650 v14 = 90.30 r29 = −31.889 d29 = 0.2r30 = −31.868 d30 = 3.2 n15 = 1.83400 v15 = 37.20 r31 = −74.308 d31 =0.2 r32 = 158.251 d32 = 8.7 n16 = 1.49700 v16 = 81.60 r33 = −62.224 d33= Variable r34 = 0.000 d34 = 93.5 n17 = 1.51680 v17 = 64.17 r35 = 0.000d35 = 10.0

TABLE 18 Projection distance 1000 2000 3000 d0  1000 2000 3000 d1276.418 75.318 75.018 d18 29.981 31.08 31.381 d33 3.978 3.45 3.299

The charts in FIG. 32 show the performance of Working Example 8 withrespect to various aberrations and it can be seen that the wide anglelens according to Working Example 8 shows favorable performance withrespect to aberration.

WORKING EXAMPLE 9

FIG. 33 is a view of a configuration of a wide angle lens according toWorking Example 9. A wide angle lens 600 shown in FIG. 33 has athree-group configuration, and is provided with a first lens group 601that has negative refractive power (lens 601 a to 601 f), a second lensgroup 602 that has weak refractive power (lens 602 a to 601 c) and athird lens group 603 that has positive refractive power (lens 603 a to603 g), in the order as seen from the side with the longer conjugatedistance. The basic configuration is the same as the wide angle lens 400shown in FIG. 29.

In Working Example 9, F_(NO)=2.5, a focal length f=14.87 and a halfangle of view of 38.7°. The values of the above noted Expressions (23)to (31) are shown below:bf/f=5.13  Expression (23)f/f1 g=−0.336  Expression (24)f/f2 g=−0.0304  Expression (25)f/f3 g=0.208  Expression (26)(1/f1 /abe1 )/(1/f3 g)=−0.0208  Expression (27)nd11=1.7847  Expression (28)nd4=1.835  Expression (29)vd4=42.98  Expression (30)f4 r/bf=1.32.  Expression (31)

Specific figures are shown below in Table 19, and zoom data is shown inTable 20. In the example of Table 19, r1 to r12 is the first lens group,r13 to r18 is the second lens group, r20 to r33 is the third lens groupand r19 is the aperture stop.

TABLE 19 ri di Nd νd r1 = 102.196 d1 = 5.7 n1 = 1.78472 v1 = 25.72 r2 =50.643 d2 = 13.0 r3 = 108.506 d3 = 4.5 n2 = 1.48749 v2 = 70.44 r4 =53.567 d4 = 15.9 r5 = 168.848 d5 = 13.5 n3 = 1.58913 v3 = 61.25 r6 =−185.570 d6 = 1.0 r7 = −998.071 d7 = 3.6 n4 = 1.49700 v4 = 81.61 r8 =47.844 d8 = 12.0 r9 = −196.997 d9 = 3.6 n5 = 1.49700 v5 = 81.61 r10 =85.511 d10 = 1.0 r11 = 51.604 d11 = 11.3 n6 = 1.74950 v6 = 35.04 r12 =87.625 d12 = Variable r13 = 640.032 d13 = 3.0 n7 = 1.61800 v7 = 63.39r14 = 40.218 d14 = 1.0 r15 = 41.292 d15 = 6.0 n8 = 1.71736 v8 = 29.50r16 = 52.118 d16 = 5.4 r17 = 98.819 d17 = 5.5 n9 = 1.58144 v9 = 40.89r18 = −98.560 d18 = Variable r19 = 0.000 d19 = 45.7 r20 = −924.647 d20 =6.3 n10 = 1.62004 v10 = 36.37 r21 = −74.737 d21 = 0.3 r22 = 236.715 d22= 7.6 n11 = 1.49700 v11 = 81.61 r23 = −55.254 d23 = 0.8 r24 = −51.097d24 = 2.6 n12 = 1.69680 v12 = 55.46 r25 = −676.690 d25 = 0.5 r26 =198.825 d26 = 3.3 n13 = 1.83500 v13 = 42.98 r27 = 44.573 d27 = 1.3 r28 =49.226 d28 = 12.0 n14 = 1.45650 v14 = 90.27 r29 = −46.103 d29 = 1.0 r30= −40.409 d30 = 3.2 n15 = 1.83400 v15 = 37.34 r31 = −74.935 d31 = 0.3r32 = 149.243 d32 = 10.0 n16 = 1.49700 v16 = 81.61 r33 = −50.445 d33 =Variable r34 = 0.000 d34 = 93.5 n17 = 1.51680 v17 = 64.17 r35 = 0.000d35 = 10.0

TABLE 20 Projection distance 1000 2000 3000 d0  1000 2000 3000 d12 73.3371.73 71.23 d18 60.90 62.50 63.00 d33 4.736 4.752 4.753

Furthermore, when X is the amount of offset from the lens apex at theposition of a radial distance h of the aperture from the optical axis,the shape of the aspherical surface is a rotatably symmetricalaspherical surface that is represented by Formula 1 below

Formula 1

$X = {\frac{h^{2}/r}{1 + \left\{ {1 - \left( {h/r} \right)^{2}} \right\}^{1/2}} + {\sum\limits_{i = 4}^{10}{{Ai} \cdot {h^{i}.}}}}$The aspherical coefficients of the surfaces are shown below Asphericalcoefficients of five facesA4=9.45575×10⁻⁰⁰⁷A6=−1.53739×10⁻⁰¹⁰A8=1.08192×10⁻⁰¹³.

The charts in FIG. 34 show the performance of Working Example 9 withrespect to the various aberrations and it can be seen that the wideangle lens according to Working Example 9 shows favorable performancewith respect to aberration.

EMBODIMENT 6

FIG. 35 is a view of a configuration of a video enlarging/projectingsystem 40 according to Embodiment 6 of the present invention. The videoenlarging/projecting system 40 is provided with a projecting lens 41that is constituted by any zoom lens or wide angle lens according toEmbodiments 1 to 5, a spatial optical modulating element 42 for formingan optical image, and a light source 43.

Numeral 44 denotes a focus surface for the image that is projected. Theoptical image that is formed by the spatial optical modulating element42 that is illuminated by the light source 43 is enlarged and projectedonto the focus surface 44.

The video enlarging projecting system 40 according to the presentembodiment uses any of the zoom lenses or wide angle lenses according tothe Embodiments 1 to 5, as the projecting lens 41, and thus it ispossible to obtain a screen that has little distortion or colorbleeding.

EMBODIMENT 7

FIG. 36 is a view of a configuration of a video projector 50 accordingto Embodiment 7 of the present invention. The video projector 50 isprovided with a projecting lens 51 that is constituted by any zoom lensor wide angle lens according to Embodiments 1 to 5, a spatial opticalmodulating element 52 for forming an optical image, rotating means 53and a light source 54.

Each of three types of color optical images, blue, green and red, istemporally separated and formed on the spatial optical modulatingelement 52. The rotating means 53 temporally restricts the optical imageto the three colors of blue, green and red by rotating a filter thatcorresponds to blue, green and red.

Light from the light source 54 is temporally broken down into the threecolors of blue, green and red by the rotating means 53, and illuminatesthe spatial optical modulating element 52. The three types of coloroptical images, blue, green and red, are temporally separated and formedon the spatial optical modulating element 52, and enlarged and projectedby the projecting lens 51.

By using a zoom lens according to any one of Embodiment 1 to 3 as theprojecting lens 51, it is possible to obtain a screen that is bright,and that has little distortion or color bleeding. By using the zoom lensof Embodiment 4, it is possible to realize a compact video projector bywhich an image that is bright and that has little distortion or imagebleeding may be obtained. By using the wide angle lens of Embodiment 5,it is possible to realize a video projector that is capable of use overa short projection distance.

EMBODIMENT 8

FIG. 37 is a view of a configuration of a rear projector 60 according toEmbodiment 8 of the present invention. The rear projector 60 is providedwith a video projector 61 in which a zoom lens or a wide angle lensaccording to any one of Embodiments 1 to 5 is used, a mirror 62 forbending light, a transmissive-type screen 63, and a casing 64.

The image projected from the video projector 60 is reflected by themirror 62, and the image is formed on the transmissive-type screen 63.When a zoom lens according to any one of Embodiments 1 to 3 is used inthe video projector 60, it is possible to realize a high definition rearprojector. When the wide angle lens according to Embodiment 4 is used,it is possible to realize a compact high definition rear projector, andwhen the wide angle lens according to Embodiment 5 is used, it ispossible to make a compact rear projector.

EMBODIMENT 9

FIG. 38 is a view of the configuration of a multivision system 70according to Embodiment 9 of the present invention. The multivisionsystem 70 shown in the diagram is provided with video projectors 71, inwhich a zoom lens or wide angle lens according to any one of Embodiments1 to 5 is used, transmissive-type screens 72, casings 73, and an imageseparation circuit 74 for separating an image.

An image signal is processed and separated by the image separationcircuit 74, and sent to a plurality of the video projectors 71. Theimage that is projected from the video projectors 71 is formed on thetransmissive-type screen 72. With the present embodiment, when a zoomlens according to any one of Embodiments 1 to 3 is used in the videoprojectors 71, it is possible to realize a multivision system in whichthe joins between the images are smooth, and which presents nounpleasant offset. When the zoom lens of Embodiment 4 is used, it ispossible to realize a multivision system in which the joins between theimages are smooth, in which there is no unpleasant offset and that iscompact. When the wide angle lens of Embodiment 5 is used, it ispossible to realize a compact rear projector.

It should be noted that Embodiments 6 to 9 have been described withexamples in which a zoom lens or a wide angle lens according to any oneof Embodiments 1 to 5 is used in the video projecting/enlarging systemand the like, however, they may also be used in optical devices such asvideo cameras, film cameras and digital cameras that form imageinformation on image-taking means such as film and CCDs.

INDUSTRIAL APPLICABILITY

According to the invention as described above, since the presentinvention has a lens that has positive refractive power on the side ofthe second lens group, which has negative refractive power, that has thelonger conjugate distance, it is possible to suppress distortionaberration to a small value. Thus, by using the zoom lens according tothe present invention, it is possible to realize a bright, highdefinition video enlarging/projecting system, video projector, rearprojector and multivision system.

1. A zoom lens comprising at least three lens groups that are arrangedin order of a first lens group that has positive refractive power, and asecond lens group that has negative refractive power, as seen from theside having the longer conjugate distance; wherein the first lens of thelenses of the second lens group as seen from the side having the longerconjugate distance has positive refractive power; wherein thearrangement of the refractive power of the lenses of the second lensgroup is one of: (a) positive, negative, negative, positive, negative or(b) positive, negative, negative, negative, positive, negative, as seenfrom the side having the longer conjugate distance; and wherein the zoomlens does not have a joined surface.
 2. The zoom lens according to claim1, wherein the following relationship is satisfied:−0.6<f2 g/f2 top<−0.15 where f2top is the focal length of a first lens,as seen from the side having the longer conjugate distance, of thelenses of the second lens group, and where f2g is the focal length ofthe second lens group.
 3. The zoom lens according to claim 1, whereinthe following relationship is satisfied:0.25<frear/f2 top<0.95 where f2top is the focal length of a first lens,as seen from the side having the longer conjugate distance, of thelenses of the second lens group, and where frear is the focal length ofthe lens group on the side having the shorter conjugate distance, withrespect to an aperture stop.
 4. The zoom lens according to claim 1,wherein the front lens, as seen from the side having the longerconjugate distance, is a negative lens, and wherein the followingrelationships are satisfied:−0.018<(1/f1 /abe1 )/(1/frear)<01.7<nd11<1.79 where f1 is the focal length of the negative lens, whereabel is the Abbe number and where nd11 is the refractive index at the dline, and where frear is the focal length of the lens group on the sidehaving the shorter conjugate distance, with respect to an aperture stop.5. The zoom lens according to claim 1, wherein four lenses, as seen fromthe side having the shorter conjugate distance. comprises: from the sidehaving the longer conjugate distance, a negative meniscus lens whoseconvex surface faces the side having the longer conjugate distance, apositive lens, a negative meniscus lens whose convex surface faces theside having the shorter conjugate distance and a positive lens, whereinthe following relationships are satisfied:nd4>1.75vd4>401<f4 r/bfw<4 where nd4 is the refractive index at the d line of thenegative meniscus lens that is on the side having the longer conjugatedistance, where vd4 is the Abbe number, where f4r is the focal length ofthe four lenses and where bfw is the air equivalent back focus that doesnot include a prism and a cover glass when at the wide angle end.
 6. Thezoom lens according to claim 1, wherein the first lens group that haspositive refractive power, the second lens group that has a negativerefractive index and the third lens group that has a positive refractiveindex, are arranged in that order from the side having the longerconjugate distance; wherein when changing magnification from the wideangle end to the telephoto end, the first lens group, the second lensgroup and the third lens group move along the optical axis; wherein thefirst lens group moves monotonically toward the side having the longerconjugate distance, the second lens group moves monotonically toward theside having the shorter conjugate distance and the third lens groupmoves monotonically toward the side having the longer conjugatedistance; and wherein the following relationship is satisfied:1.6<bfw/fw<2.4 where bfw is the air equivalent back focus of the zoomlens at the wide angle end when at infinity and where fw is the focallength of the zoom lens at the wide angle end.
 7. The zoom lensaccording to claim 6, wherein the following relationships are satisfied:0,05<fw/f1 g<0.2−0.9<fw/f2 g<−0.60.5<fw/f3 g<0.7 where f1g is the focal length of the first lens group,where f2g is the focal length of the second lens group, where f3g is thefocal length of the third lens group, and where fw is the focal lengthof the zoom lens at the wide angle end.
 8. The zoom lens according toclaim 1, wherein the first lens group that has positive refractivepower, the second lens group that has a negative refractive index andthe third lens group that has a positive refractive index, are arrangedin that order from the side having the longer conjugate distance;wherein when changing magnification from the wide angle end to thetelephoto end, the first lens group, the second lens group and the thirdlens group move along the optical axis; wherein the first lens groupmoves monotonically toward the side having the longer conjugatedistance, the second lens group moves monotonically toward the sidehaving the shorter conjugate distance and the third lens group movesmonotonically toward the side having the longer conjugate distance; andwherein the following relationship is satisfied:1<bfw/fw<1.8 where bfw is the air equivalent back focus of the zoom lensat the wide angle end when at infinity and where fw is the focal lengthof the zoom lens at the wide angle end.
 9. The zoom lens according toclaim 8, wherein the following relationships are satisfied:0.3<fw/f1 g<0.4−1.6<fw/f2 g<1.30.7<fw/f3 g<0.9 where f1g is the focal length of the first lens group,where f2g is the focal length of the second lens group, where f3g is thefocal length of the third lens group, and where fw is the focal lengthof the zoom lens at the wide angle end.
 10. The zoom lens according toclaim 1, comprising the first lens group that has positive refractivepower, the second lens group that has a negative refractive index and athird lens group that has a positive refractive index, arranged in thatorder from the side having the longer conjugate distance; wherein whenchanging magnification from the wide angle end to the telephoto end, thefirst lens group, the second lens group and the third lens group movealong the optical axis; wherein the first lens group moves monotonicallytoward the side having the longer conjugate distance, the second lensgroup moves monotonically toward the side having the shorter conjugatedistance and the third lens group moves monotonically toward the sidehaving the longer conjugate distance; and wherein the followingrelationship is satisfied:0.5<bfw/fw<1.3 where bfw is the air equivalent back focus of the zoomlens at the wide angle end when at infinity and where fw is the focallength of the zoom lens at the wide angle end.
 11. The zoom lensaccording to claim 10, wherein the following relationships aresatisfied:0.45<fw/f1 g<0.6−2.0<fw/f2 g<−1.60.9<fw/f3 g<1.3 where f1g is the focal length of the first lens group,where f2g is the focal length of the second lens group, where f3g is thefocal length of the third lens group, and where fw is the focal lengthof the zoom lens at the wide angle end.
 12. The zoom lens according toclaim 1, wherein the Abbe number of all lenses having positiverefractive power that are arranged on the side having the shorterconjugate distance with respect to an aperture stop is at least
 80. 13.The zoom lens according to claim 1, wherein the Abbe number of alllenses having negative refractive power that are arranged on the sidehaving the shorter conjugate distance with respect to an aperture stopis at least
 35. 14. The zoom lens according to claim 1, wherein thefirst lens group that has positive refractive power, the second lensgroup that has a negative refractive index and the third lens group thathas a positive refractive index, arranged in that order from the sidehaving the longer conjugate distance; wherein when changingmagnification from the wide angle end to the telephoto end, the firstlens group, the second lens group and the third lens group move alongthe optical axis; wherein the first lens group moves monotonicallytoward the side having the longer conjugate distance, the second lensgroup moves monotonically toward the side having the shorter conjugatedistance and the third lens group moves monotonically toward the sidehaving the longer conjugate distance and an aperture stop moves inconjunction with the third lens group; and wherein the followingrelationship is satisfied:|(DG1 −DG3 )/fw|<0.15 where DG1 is the amount that the first lens groupmoves from the wide angle end to the telephoto end, where DG3 is theamount that the third lens group moves from the wide angle end to thetelephoto end and where fw is the focal length of the zoom lens at thewide angle end.
 15. The zoom lens according to claim 1, wherein thefirst lens group that has positive refractive power, the second lensgroup that has a negative refractive index and the third lens group thathas a positive refractive index, arranged in that order from the sidehaving the longer conjugate distance; wherein when changingmagnification from the wide angle end to the telephoto end, the firstlens group is fixed, and the second lens group and the third lens groupmove along the optical axis; wherein the second lens group movesmonotonically toward the side having the shorter conjugate distance andthe third lens group moves monotonically toward the side having thelonger conjugate distance and an aperture stop moves in conjunction withthe third lens group; and wherein the following relationship issatisfied:|DG3/ fw|<0.15 where DG3 is the amount that the third lens group movesfrom the wide angle end to the telephoto end and where fw is the focallength of the zoom lens at the wide angle end.
 16. The zoom lensaccording to claim 1, wherein the zoom lens is a projecting lens for aprojector.
 17. The zoom lens according to claim 1, wherein themagnification ratio of the entire lens system is used in a range of−0.00058 times to −0.0188 times.
 18. The zoom lens according to claim 1,wherein the F number is 2.5 or 2.4.
 19. The zoom lens according to claim1, wherein the zoom ratio is 1.5, 1.6 or 1.65.
 20. A videoenlarging/projecting system comprising: a projecting lens in which thezoom lens according to claim 1 is used; a light source, and a spatialoptical modulating element that is illuminated by light irradiated fromthe light source, and that forms an optical image, wherein theprojecting lens projects the optical image that is formed on the spatialoptical modulating element.
 21. A video projector comprising: aprojecting lens in which the zoom lens according to claim 1 is used; alight source; means for temporally restricting light from the lightsource to three colors of blue, green and red, and a spatial opticalmodulating element that is ilhuninated by light irradiated from thelight source, and that forms an optical image that corresponds to threecolors of blue, green and red that temporally change.
 22. A rearprojector comprising: a video projector according to claim 21, a mirrorthat bends light that is projected from a projecting lens, and atransmissive-type screen for reflecting an image of projected light. 23.A multivision system comprising: a plurality of systems comprising: avideo projector according to claim 21, a transmissive-type screen forreflecting an image of projected light, and a casing; and furthercomprising an image separating circuit for separating images.